[bugfix] Warn for unset variables only for the tests that are selected

[vkarak]

The fix turned out to be easy, since we were already storing the unset variables. This is now moved at the loader level and we warn only after all test filtering has happened in the cli (but before the treatment of the special options --failed, --distribute and --repeat).

Fixes #2672.

[codecov-commenter]

Codecov Report

Base: 86.61% // Head: 86.61% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (b9a2cc3) compared to base (def32e3).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2774   +/-   ##
=======================================
  Coverage   86.61%   86.61%           
=======================================
  Files          60       60           
  Lines       11214    11218    +4     
=======================================
+ Hits         9713     9717    +4     
  Misses       1501     1501           

Impacted Files	Coverage Δ
reframe/frontend/cli.py	73.84% <100.00%> (+0.16%)	⬆️
reframe/frontend/loader.py	96.02% <100.00%> (ø)

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