[bugfix] Do not evaluate deferred expressions when dumping in JSON

[vkarak]

We instead cache the last evaluated value and returned that one. If the deferred expression has never been evaluated, then None is returned.

This addresses #1646.

[codecov-io]

Codecov Report

Merging #1665 (1e13812) into master (01d7efc) will decrease coverage by 0.30%.
The diff coverage is 49.15%.

@@            Coverage Diff             @@
##           master    #1665      +/-   ##
==========================================
- Coverage   87.54%   87.23%   -0.31%     
==========================================
  Files          45       45              
  Lines        7492     7546      +54     
==========================================
+ Hits         6559     6583      +24     
- Misses        933      963      +30     

Impacted Files	Coverage Δ
reframe/core/modules.py	56.62% <45.28%> (-1.21%)	⬇️
reframe/core/deferrable.py	97.56% <83.33%> (+0.01%)	⬆️
reframe/core/pipeline.py	92.32% <0.00%> (-0.17%)	⬇️

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