[feat] Add test API methods for skipping tests

[vkarak]

This PR adds two RegressionTest methods that allow users to skip tests dynamically:

• skip(msg=None): Skip test uncondintionally.
• skip_if(cond, msg=None): Skip test if cond is true.

A test can be skipped anywhere during its lifetime, except during the cleanup stage.
NOTE: If a test is skipped after its stage directory is created, it will not be cleaned up.

Fixes #1474.

[codecov-io]

Codecov Report

Merging #1875 (5a46cde) into master (1c9b188) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1875      +/-   ##
==========================================
+ Coverage   87.83%   87.92%   +0.08%     
==========================================
  Files          49       49              
  Lines        8395     8448      +53     
==========================================
+ Hits         7374     7428      +54     
+ Misses       1021     1020       -1     

Impacted Files	Coverage Δ
reframe/core/decorators.py	97.05% <100.00%> (-0.03%)	⬇️
reframe/core/exceptions.py	94.24% <100.00%> (+0.04%)	⬆️
reframe/core/pipeline.py	92.03% <100.00%> (+0.06%)	⬆️
reframe/frontend/executors/__init__.py	98.30% <100.00%> (+0.09%)	⬆️
reframe/frontend/executors/policies.py	99.69% <100.00%> (+0.02%)	⬆️
reframe/frontend/statistics.py	95.45% <100.00%> (+0.14%)	⬆️
reframe/frontend/printer.py	95.65% <0.00%> (+2.17%)	⬆️

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