[feat] Deprecate test syntax that overrides RegressionTest methods

[ekouts]

This feature is introduced to discourage users to override RegressionTest methods and use the hooks instead. If it is necessary, the user can still override the original methods but he has to use a new decorator that is also introduced in this pr.

Fixes #1130

Things to be done:

• implement feature
• unittests for the new feature
• fixing the original unittests, so that they don't throw warnings
• update documentation

[pep8speaks]

Hello @ekouts, 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 2020-04-14 16:15:23 UTC

[vkarak]

Moving to next sprint; it's still a wip.

[ekouts]

@jenkins-cscs retry daint kesch

[vkarak]

We will need to update the documentation as well.

[codecov-io]

Codecov Report

Merging #1228 into master will decrease coverage by 0.64%.
The diff coverage is 82.29%.

@@            Coverage Diff             @@
##           master    #1228      +/-   ##
==========================================
- Coverage   92.00%   91.35%   -0.65%     
==========================================
  Files          83       84       +1     
  Lines       12166    12495     +329     
==========================================
+ Hits        11193    11415     +222     
- Misses        973     1080     +107     

Impacted Files	Coverage Δ
unittests/test_loader.py	83.67% <70.37%> (-16.33%)	⬇️
reframe/core/pipeline.py	92.70% <94.11%> (+0.33%)	⬆️
reframe/core/meta.py	100.00% <100.00%> (ø)
unittests/resources/checks/frontend_checks.py	99.22% <100.00%> (+<0.01%)	⬆️
...ittests/resources/checks_unlisted/kbd_interrupt.py	100.00% <100.00%> (ø)
reframe/core/config.py	51.23% <0.00%> (-33.26%)	⬇️
reframe/core/schedulers/torque.py	33.33% <0.00%> (-1.81%)	⬇️
unittests/test_schedulers.py	95.37% <0.00%> (-1.49%)	⬇️
reframe/core/schedulers/slurm.py	53.51% <0.00%> (-1.32%)	⬇️
reframe/core/schedulers/pbs.py	67.85% <0.00%> (-0.82%)	⬇️
... and 15 more

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[vkarak]

lgtm now. I will update the documentation and merge it.