Every time a gem is not found in the Compact Index API, RubyGems will
fallback to the full index, which is very slow. This is unnecessary
because both indexes should be providing the same gems, so if a gem
can't be found in the Compact Index API, it won't be found in the full
index.

We _do_ want a fallback to the full index, whenever the Compact Index
API is not implemented. To detect that, we check that the API responds
to the "/versions" endpoint, just like Bundler does.

Before:

```
$ time gem install fooasdsfafs
ERROR:  Could not find a valid gem 'fooasdsfafs' (>= 0) in any repository
gem  20,77s user 0,59s system 96% cpu 22,017 total
```

After:

```
$ time gem install fooasdsfafs
ERROR:  Could not find a valid gem 'fooasdsfafs' (>= 0) in any repository
gem  5,02s user 0,09s system 91% cpu 5,568 total
```

https://github.com/rubygems/rubygems/commit/c0d6b9eea7

https://github.com/rubygems/rubygems/commit/99d91c9ed2

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

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