from https://github.com/rubygems/rubygems/commit/bfb0ae69776069155d2092702bfbb5a12617d85a

  Pick from https://github.com/rubygems/rubygems/commit/8331e63263081a6aa690d8025d2957f30c4e814a

https://github.com/rubygems/rubygems/commit/a4ba1a4d97

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Mitigate the security risk:
https://devcraft.io/2021/01/07/universal-deserialisation-gadget-for-ruby-2-x-3-x.html

https://github.com/rubygems/rubygems/commit/141c2f4388

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
59c682097197fee4052b47e4b4ab86562f3eaa9b, 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.

https://github.com/rubygems/rubygems/commit/6460e018df

https://github.com/rubygems/rubygems/commit/c77868a555

https://github.com/rubygems/rubygems/commit/769e87f011