[feat] Support relative imports in tests

[vkarak]

Relative imports in tests are currently supported only for tests inside the reframe's installation prefix. This is problematic for custom utilities and libraries that users may want to use in their tests.

To enable relative imports in tests loaded by reframe, the parent directory of the test must contain a __init__.py file.

I've updated the structure of the existing test library in the unit tests to expose the limitation of the current implementation and added a unit test that reproduces it.

Closes #3076.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f2a2326) 86.64% compared to head (0662303) 86.64%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3110   +/-   ##
========================================
  Coverage    86.64%   86.64%           
========================================
  Files           61       61           
  Lines        12046    12051    +5     
========================================
+ Hits         10437    10442    +5     
  Misses        1609     1609           

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