Revert #3445

  These changes completely broke the test suite on ruby core repository.

lib/rubygems/test_case.rb

@@ -11,7 +11,10 @@
   Gem::Specification.dirs.shift
 end
 
-gem 'minitest', '~> 5.13'
+begin
+  gem 'minitest', '~> 5.13'
+rescue Gem::LoadError
+end
 
 begin
   require 'simplecov'
@@ -95,7 +98,7 @@ def self.specific_extra_args_hash=(value)
 # and uninstall gems, fetch remote gems through a stub fetcher and be assured
 # your normal set of gems is not affected.
 
-class Gem::TestCase < Minitest::Test
+class Gem::TestCase < (defined?(Minitest::Test) ? Minitest::Test : MiniTest::Unit::TestCase)
 
   extend Gem::Deprecate
 
@@ -124,6 +127,11 @@ def assert_activate(expected, *specs)
     assert_equal expected.sort, loaded.sort if expected
   end
 
+  def assert_path_exists(path, msg = nil)
+    msg = message(msg) { "Expected path '#{path}' to exist" }
+    assert File.exist?(path), msg
+  end
+
   def assert_directory_exists(path, msg = nil)
     msg = message(msg) { "Expected path '#{path}' to be a directory" }
     assert_path_exists path
@@ -217,6 +225,11 @@ def exeext(value)
     end
   end
 
+  def refute_path_exists(path, msg = nil)
+    msg = message(msg) { "Expected path '#{path}' to not exist" }
+    refute File.exist?(path), msg
+  end
+
   def scan_make_command_lines(output)
     output.scan(/^#{Regexp.escape make_command}(?:[[:blank:]].*)?$/)
   end