[feat] Support custom indexed and external references

[vkarak]

This PR extends the test's reference syntax to support arbitrary indexing on test attributes as well as other configuration parameters, such as the current partition, environment, processor and/or device details. If no special indexing is requested, the reference dictionary behaves exactly as before.

Support for external reference files is also added, where the reference dict of a test can be in an external YAML file.

Check the docs for all the details of the new features.

Implementation details

The test's prefix is now passed as a metaclass parameter and is not set during the test's initialization. This was needed to support the default external reference prefix, which coincides with the test's prefix, so we needed the test's prefix to be available during class creation and when the reference is defined in the test class body.
Since the test prefix is known during the class creation it is more natural to move there. Also all prefix manipulations are handled in single place now.

Closes #3463

[codecov]

Codecov Report

❌ Patch coverage is 95.94595% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.70%. Comparing base (c9fa4c7) to head (4f236d4).
⚠️ Report is 7 commits behind head on develop.

Files with missing lines	Patch %	Lines
reframe/core/pipeline.py	96.01%	8 Missing ⚠️
reframe/frontend/testgenerators.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3630      +/-   ##
===========================================
+ Coverage    91.64%   91.70%   +0.06%     
===========================================
  Files           62       62              
  Lines        13530    13713     +183     
===========================================
+ Hits         12399    12576     +177     
- Misses        1131     1137       +6     

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[victorusu]

lgtm

[vkarak]

Giving it a final testing round on our actual tests, I discovered a bug. If a reference with external references is set in the test class body, it crashes. This was not caught by the unit tests because, they were setting the reference after the test instantiation. I changed the unit tests to reproduce the error, and now working on fixing it.

[vkarak]

Still not merging; I discovered a bug in the prefix calculation when the make_test() is used. I'm adding a unit test to reproduce it.