[bugfix] Capture all pending test tasks in the exit loop of the asynchronous execution policy

[ekouts]

Besides _running_tasks and _waiting_tasks, we can have a more strict check in the async policy main loop. In this way we make sure that the tasks will run in the correct reframe "retry".

I couldn't find how to reproduce the bug, but it should fix #1398 .

[codecov-commenter]

Codecov Report

Merging #1434 into master will decrease coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
- Coverage   91.83%   91.54%   -0.30%     
==========================================
  Files          82       82              
  Lines       12798    12365     -433     
==========================================
- Hits        11753    11319     -434     
- Misses       1045     1046       +1     

Impacted Files	Coverage Δ
reframe/frontend/executors/policies.py	98.23% <100.00%> (+0.01%)	⬆️
...tests/resources/checks_unlisted/deprecated_test.py	53.84% <0.00%> (-3.30%)	⬇️
reframe/core/systems.py	86.06% <0.00%> (-2.13%)	⬇️
reframe/core/modules.py	60.51% <0.00%> (-1.93%)	⬇️
unittests/test_launchers.py	93.54% <0.00%> (-1.39%)	⬇️
unittests/test_environments.py	72.07% <0.00%> (-1.22%)	⬇️
reframe/core/deferrable.py	96.62% <0.00%> (-0.88%)	⬇️
reframe/core/exceptions.py	85.07% <0.00%> (-0.75%)	⬇️
reframe/core/environments.py	95.00% <0.00%> (-0.66%)	⬇️
reframe/core/schedulers/slurm.py	56.94% <0.00%> (-0.61%)	⬇️
... and 35 more

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[ekouts]

@jenkins-cscs retry all

[vkarak]

lgtm