[CIVIS-9349] fix minitest instrumentation with mixins

[anmarchenko]

What does this PR do?
Fixes #130.

Uses Ruby's Module.const_source_location method that is available since Ruby 2.7 to determine the source location of Minitest::Runnable subclass (test suite).

Motivation
Bug reported in the linked issue shows that we did not pay enough attention to the fact that Minitest allows tests to be defined in shared modules and be mixed in any test suite. The current way to determine test suite's source location is brittle because it returns method's location which could be anything in this case. This worked fine for tests that didn't use mixins and for tests that used only mixins, but broke for tests that used both mixed in tests and normal tests.

Proposed fix is to use where the class constant is defined as test suite location (and method only as fallback). As we target Ruby 2.7+ support since GA, it is safe to use and works for all cases.

Tests
Tests using ActiveSupport and mixins are provided

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.25926% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 99.08%. Comparing base (f71d45c) to head (570e56c).

Files	Patch %	Lines
...g/ci/contrib/minitest_shoulda_context/fake_test.rb	96.15%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #134    +/-   ##
========================================
  Coverage   99.08%   99.08%            
========================================
  Files         156      164     +8     
  Lines        7102     7236   +134     
  Branches      300      304     +4     
========================================
+ Hits         7037     7170   +133     
- Misses         65       66     +1     

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