[feat] Unify the names of pre/postrun and pre/postbuild cmds

[teojgo]

Fixes #1269

[vkarak]

Lgtm except that we should still keep the old attributes and deprecate them! With fields it's pretty simple to do.

[vkarak]

I will fix these. I will do some minor fixes to the documentation as well.

[codecov-commenter]

Codecov Report

Merging #1336 into master will increase coverage by 0.00%.
The diff coverage is 96.15%.

@@           Coverage Diff           @@
##           master    #1336   +/-   ##
=======================================
  Coverage   91.78%   91.78%           
=======================================
  Files          83       83           
  Lines       12561    12565    +4     
=======================================
+ Hits        11529    11533    +4     
  Misses       1032     1032           

Impacted Files	Coverage Δ
reframe/utility/color.py	100.00% <ø> (ø)
reframe/core/pipeline.py	92.89% <92.85%> (+0.05%)	⬆️
unittests/resources/checks/frontend_checks.py	99.34% <100.00%> (ø)
unittests/resources/checks_unlisted/good.py	87.87% <100.00%> (ø)
unittests/test_pipeline.py	94.08% <100.00%> (ø)

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[vkarak]

@teojgo After we merged with master, the unit tests fail because of two deprecation warnings:

=============================================================================================== warnings summary ===============================================================================================
unittests/test_policies.py::test_compile_fail_reschedule_main_loop
unittests/test_policies.py::test_compile_fail_reschedule_busy_loop
  /Users/karakasv/Repositories/reframe/unittests/resources/checks/frontend_checks.py:232: ReframeDeprecationWarning: 'prebuild_cmd' is deprecated; please use 'prebuild_cmds' instead
    self.prebuild_cmd = ['echo foo > x.c']

-- Docs: https://docs.pytest.org/en/latest/warnings.html

[vkarak]

We need to fix deprecation warnings in unit tests.