[feat] Skip test if sanity_patterns is undefined

[vkarak]

It is a common mistake to forget to define the sanity_patterns. Now tests not defining them, will be skipped and a warning message will be issued, similarly to valid_systems and valid_prog_environs.

[codecov-io]

Codecov Report

Merging #1876 (d367f1d) into master (ed40b22) will increase coverage by 0.23%.
The diff coverage is 94.98%.

@@            Coverage Diff             @@
##           master    #1876      +/-   ##
==========================================
+ Coverage   87.61%   87.85%   +0.23%     
==========================================
  Files          49       49              
  Lines        8141     8405     +264     
==========================================
+ Hits         7133     7384     +251     
- Misses       1008     1021      +13     

Impacted Files	Coverage Δ
reframe/core/variables.py	94.83% <93.93%> (-1.64%)	⬇️
reframe/core/exceptions.py	94.59% <100.00%> (+0.39%)	⬆️
reframe/core/meta.py	100.00% <100.00%> (ø)
reframe/core/namespaces.py	95.00% <100.00%> (+1.89%)	⬆️
reframe/core/parameters.py	98.50% <100.00%> (+0.04%)	⬆️
reframe/core/pipeline.py	91.96% <100.00%> (-0.02%)	⬇️
reframe/frontend/loader.py	92.50% <100.00%> (ø)

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