[rubygems/rubygems] Rename test/rubygems/test_{case,utilities}.rb to avoid "test_" prefix

This changes "test/rubygems/test_case.rb" to "test/rubygems/helper.rb",
and "test/rubygems/test_utilities.rb" to "test/rubygems/utilities.rb".

The two files are a helper for tests, not test files. However, a file
starting with "test_" prefix is handled as a test file directly loaded
by test-unit because Rakefile specifies:

```
t.test_files = FileList['test/**/test_*.rb']
```

Directly loading test/rubygems/test_utilities.rb caused "uninitialized
constant Gem::TestCase". This issue was fixed by
59c6820971, but the fix caused a
"circular require" warning because test_utilities.rb and test_case.rb
are now requiring each other.

Anyway, adding "test_" prefix to a test helper file is confusing, so
this changeset reverts the fix and solve the issue by renaming them.

6460e018df

test/rubygems/test_gem_commands_outdated_command.rb

@@ -1,5 +1,5 @@
 # frozen_string_literal: true
-require_relative 'test_case'
+require_relative 'helper'
 require 'rubygems/commands/outdated_command'
 
 class TestGemCommandsOutdatedCommand < Gem::TestCase