[bugfix] Treat correctly forced local jobs and other small fixes

[vkarak]

Fixes #1511.

This PR also addresses the following:

• Fix System JSON representation
• Adjust sleep times in local scheduler unit tests

[victorusu]

lgtm

[codecov-io]

Codecov Report

Merging #1513 into master will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1513      +/-   ##
==========================================
+ Coverage   91.51%   91.55%   +0.03%     
==========================================
  Files          83       83              
  Lines       13198    13224      +26     
==========================================
+ Hits        12078    12107      +29     
+ Misses       1120     1117       -3     

Impacted Files	Coverage Δ
reframe/core/systems.py	88.31% <ø> (ø)
reframe/frontend/executors/__init__.py	98.69% <100.00%> (+<0.01%)	⬆️
reframe/frontend/executors/policies.py	99.32% <100.00%> (+0.03%)	⬆️
unittests/test_cli.py	91.13% <100.00%> (+0.16%)	⬆️
unittests/test_policies.py	99.50% <100.00%> (+<0.01%)	⬆️
unittests/test_schedulers.py	94.25% <100.00%> (ø)
reframe/core/schedulers/slurm.py	54.03% <0.00%> (+0.83%)	⬆️

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