[feat] Support multiple tags in extract* sanity functions

[teojgo]

• Support also passing of multiple conversion functions for
  each tag to be extracted.

Fixes #1245

[teojgo]

@vkarak Does this cover our case? If yes I will also add the documentation.

[codecov-commenter]

Codecov Report

Merging #1433 into master will increase coverage by 0.10%.
The diff coverage is 90.27%.

@@            Coverage Diff             @@
##           master    #1433      +/-   ##
==========================================
+ Coverage   91.83%   91.93%   +0.10%     
==========================================
  Files          82       82              
  Lines       12798    12959     +161     
==========================================
+ Hits        11753    11914     +161     
  Misses       1045     1045              

Impacted Files	Coverage Δ
reframe/utility/sanity.py	97.45% <82.92%> (-1.50%)	⬇️
unittests/test_sanity_functions.py	100.00% <100.00%> (ø)
reframe/core/pipeline.py	92.56% <0.00%> (-0.15%)	⬇️
reframe/frontend/executors/policies.py	98.23% <0.00%> (+0.01%)	⬆️
unittests/test_policies.py	98.71% <0.00%> (+0.05%)	⬆️
unittests/test_cli.py	90.97% <0.00%> (+0.16%)	⬆️
reframe/frontend/cli.py	79.44% <0.00%> (+0.91%)	⬆️
reframe/frontend/statistics.py	95.62% <0.00%> (+4.02%)	⬆️
reframe/core/exceptions.py	90.07% <0.00%> (+4.25%)	⬆️

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[vkarak]

@teojgo We will also need to update the documentation.

[teojgo]

@teojgo We will also need to update the documentation.

Yes I wanted first to see if what I thought about the tag/conv function was correct.

[vkarak]

Just two very minor things still.