[feat] Remove support for the very old test syntax using _get_checks()

[vkarak]

I have also adapted the OpenFOAM tests to the new syntax, since they were the only ones remaining to be ported. @kraushm Can you have a look into them?

This PR renders unnecessary the call to super().__init__() inside the user test's constructor.

[pep8speaks]

Hello @vkarak, 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 2019-07-19 15:09:57 UTC

[vkarak]

@victorusu @teojgo This PR is a requirement for the stuff I'm doing with the delayed initialization of tests.

[vkarak]

I am converting it to wip, because I still need to remove the ability to pass arguments to the RegressionTest's __init__(), which was only meaningful with the old syntax.

[vkarak]

With this PR it will no more be necessary to call super().__init__() inside the user tests. Should I change all the tutorial example?

[teojgo]

@vkarak yes change them and remove super().__init__() from the OpenFoam tests included here.

[codecov-io]

Codecov Report

Merging #876 into master will increase coverage by 0.02%.
The diff coverage is 98.21%.

@@            Coverage Diff             @@
##           master     #876      +/-   ##
==========================================
+ Coverage   91.66%   91.69%   +0.02%     
==========================================
  Files          80       78       -2     
  Lines       10318    10282      -36     
==========================================
- Hits         9458     9428      -30     
+ Misses        860      854       -6

Impacted Files	Coverage Δ
...s/resources/checks_unlisted/dependencies/normal.py	100% <ø> (ø)	⬆️
unittests/test_loader.py	100% <ø> (ø)	⬆️
unittests/resources/checks/hellocheck.py	100% <ø> (ø)	⬆️
unittests/resources/checks/frontend_checks.py	99.14% <ø> (-0.01%)	⬇️
unittests/test_utility.py	99.2% <ø> (-0.01%)	⬇️
unittests/resources/checks_unlisted/good.py	87.87% <ø> (-0.7%)	⬇️
...ittests/resources/checks_unlisted/kbd_interrupt.py	100% <100%> (ø)	⬆️
reframe/core/pipeline.py	93.52% <100%> (+0.01%)	⬆️
unittests/resources/checks/bad/invalid_check.py	100% <100%> (ø)	⬆️
unittests/test_pipeline.py	97.44% <100%> (+0.04%)	⬆️
... and 4 more

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