Set @tempdir as early as possible

We have seen some errors during test `setup` before `@tempdir` is
actually set, in particular, when instantiating `Gem::MockUI` which in
turn requires `stringio`. In this case, the `teardown` method will also
produce an error because it tries to remove the `@tempdir`, which has
not been set, so tries to remove a `nil` directory. It results in a
confusing error like

```
<internal:D:/a/rubygems/rubygems/lib/rubygems/core_ext/kernel_require.rb>:85:in `require': Interrupt
Error: test_bump_one_level(TestGemVersion): TypeError: no implicit conversion of nil into String
C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/3.1.0/fileutils.rb:1570:in `path'
	from <internal:D:/a/rubygems/rubygems/lib/rubygems/core_ext/kernel_require.rb>:85:in `require'
C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/3.1.0/fileutils.rb:1570:in `block in fu_list'
	from D:/a/rubygems/rubygems/lib/rubygems/mock_gem_ui.rb:43:in `initialize'
C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/3.1.0/fileutils.rb:1570:in `map'
	from D:/a/rubygems/rubygems/test/rubygems/helper.rb:323:in `new'
C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/3.1.0/fileutils.rb:1570:in `fu_list'
	from D:/a/rubygems/rubygems/test/rubygems/helper.rb:323:in `setup'
C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/3.1.0/fileutils.rb:616:in `rm_r'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/fixture.rb:284:in `run_fixture_callback'
C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/3.1.0/fileutils.rb:638:in `rm_rf'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/fixture.rb:272:in `block in create_fixtures_runner'
D:/a/rubygems/rubygems/test/rubygems/helper.rb:462:in `teardown'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/fixture.rb:276:in `block in create_fixtures_runner'
===============================================================================
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/fixture.rb:257:in `run_fixture'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/fixture.rb:292:in `run_setup'
Finished in 683.0312862 seconds.
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testcase.rb:564:in `block in run'
-------------------------------------------------------------------------------
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testcase.rb:563:in `catch'
2185 tests, 5929 assertions, 0 failures, 1 errors, 28 pendings, 0 omissions, 0 notifications
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testcase.rb:563:in `run'
98.7185% passed
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testsuite.rb:124:in `run_test'
-------------------------------------------------------------------------------
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testsuite.rb:53:in `run'
3.20 tests/s, 8.68 assertions/sTerminate batch job (Y/N)?
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testsuite.rb:124:in `run_test'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testsuite.rb:53:in `run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testsuite.rb:124:in `run_test'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/testsuite.rb:53:in `run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnermediator.rb:67:in `run_suite'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnermediator.rb:45:in `block (2 levels) in run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnermediator.rb:102:in `with_listener'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnermediator.rb:41:in `block in run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnermediator.rb:39:in `catch'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnermediator.rb:39:in `run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunner.rb:40:in `start_mediator'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunner.rb:25:in `start'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/ui/testrunnerutilities.rb:24:in `run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/autorunner.rb:458:in `block in run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/autorunner.rb:514:in `change_work_directory'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/autorunner.rb:457:in `run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit/autorunner.rb:66:in `run'
	from C:/hostedtoolcache/windows/Ruby/3.1.1/x64/lib/ruby/gems/3.1.0/gems/test-unit-3.5.3/lib/test/unit.rb:518:in `block (2 levels) in <top (required)>'
rake aborted!
Interrupt:
```

The should make this same error much cleaner if it happens again.

test/rubygems/helper.rb

@@ -308,6 +308,9 @@ def setup
 
     FileUtils.mkdir_p @tmp
 
+    @tempdir = Dir.mktmpdir("test_rubygems_", @tmp)
+    @tempdir.tap(&Gem::UNTAINT)
+
     ENV['GEM_VENDOR'] = nil
     ENV['GEMRC'] = nil
     ENV['XDG_CACHE_HOME'] = nil
@@ -325,9 +328,6 @@ def setup
     # capture output
     Gem::DefaultUserInteraction.ui = Gem::MockGemUi.new
 
-    @tempdir = Dir.mktmpdir("test_rubygems_", @tmp)
-    @tempdir.tap(&Gem::UNTAINT)
-
     ENV["TMPDIR"] = @tempdir
 
     @orig_SYSTEM_WIDE_CONFIG_FILE = Gem::ConfigFile::SYSTEM_WIDE_CONFIG_FILE