[feat] Refactor test registry

[jjotero]

The test registry is now a class that the loader can interact with. This TestRegistry class has a public member function instantiate_all which is equivalent to _rfm_gettests() in the previous implementation. This should give the loader far more control over the test instantiation and simplify the implementation of #2117.

However, the parameterized_test decorator deals with the construction arguments in a different way and it's incompatible with this new registry. In any case, since this decorator is deprecated, we can keep the current test registry as a "legacy_registry" which is only used by tests decorated as parameterized_test.

Similarly, the deprecated require_version decorator also complicates things slightly, forcing the TestRegistry class to account for the tests to be skipped. All this extra logic can be trimmed out once the require_version decorator is dropped.

[pep8speaks]

Hello @jjotero, Thank you for updating!

Cheers! There are no PEP8 issues in this Pull Request!Do see the ReFrame Coding Style Guide

Comment last updated at 2021-08-12 17:58:24 UTC

[codecov-commenter]

Codecov Report

Merging #2119 (4e155ce) into master (4c4de13) will decrease coverage by 0.30%.
The diff coverage is 80.76%.

❗ Current head 4e155ce differs from pull request most recent head a867698. Consider uploading reports for the commit a867698 to get more accurate results

@@            Coverage Diff             @@
##           master    #2119      +/-   ##
==========================================
- Coverage   86.26%   85.96%   -0.31%     
==========================================
  Files          53       53              
  Lines        9313     9355      +42     
==========================================
+ Hits         8034     8042       +8     
- Misses       1279     1313      +34     

Impacted Files	Coverage Δ
reframe/core/decorators.py	50.33% <78.26%> (-14.16%)	⬇️
reframe/frontend/loader.py	93.70% <100.00%> (+1.57%)	⬆️

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[vkarak]

Lgtm. Just a couple of minor comments.