[enhancement] Warn on unset variables in fixtures when using the -S option

[vkarak]

Closes #2757.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f01551f) 87.35% compared to head (a33cda4) 87.35%.

❗ Current head a33cda4 differs from pull request most recent head 839b354. Consider uploading reports for the commit 839b354 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2948   +/-   ##
=======================================
  Coverage   87.35%   87.35%           
=======================================
  Files          60       60           
  Lines       11677    11683    +6     
=======================================
+ Hits        10200    10206    +6     
  Misses       1477     1477           

Impacted Files	Coverage Δ
reframe/core/decorators.py	95.95% <100.00%> (+0.21%)	⬆️
reframe/frontend/cli.py	82.19% <100.00%> (ø)
reframe/frontend/loader.py	94.59% <100.00%> (+0.03%)	⬆️

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