Module#parent_name broken for frozen classes

[coreyward]

Previously memoization inside of Module.parent_name resulted in a runtime error when called for the first time on a frozen module. This patch adds a regression test and refactors the method to resolve the issue.

Note: introspection tests were previously lumped into test/core_ext/module_test.rb despite the code having been extracted to core_exts/module/introspection.rb. I isolated the tests similarly, following convention (e.g., core_exts/module/anonymous.rb to test/core_ext/module/anonymous_test.rb). In the process, I removed some old (over 10 years) setup objects that were no longer being referenced.

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[maclover7]

ref #27637

[maclover7]

Can you squash down to one commit? Otherwise looks good to me. Thanks for working on this!

[coreyward]

Done!

[rafaelfranca]

Can we keep the if/else where. We prefer it over short-circuit returns.

[coreyward]

@rafaelfranca Updated to reflect your concerns.

Cheers!

[rafaelfranca]

Backported 9f4fc67