test/rubygems/test_gem_dependency_installer.rb: Avoid Dir.chdir + block

This caused a warning "conflicting chdir during another chdir block"
during "make test-all".

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63658 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/rubygems/test_gem_dependency_installer.rb

@@ -445,9 +445,13 @@ def test_install_dependency_existing_extension
     FileUtils.mv f1_gem, @tempdir
     inst = nil
 
-    Dir.chdir @tempdir do
+    pwd = Dir.getwd
+    Dir.chdir @tempdir
+    begin
       inst = Gem::DependencyInstaller.new
       inst.install 'f'
+    ensure
+      Dir.chdir pwd
     end
 
     assert_equal %w[f-1], inst.installed_gems.map { |s| s.full_name }