[bugfix] Fix unit test failure of asynchronous execution policy

[vkarak]

Requires #1410.
Fixes #1369.

[codecov-commenter]

Codecov Report

Merging #1411 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1411      +/-   ##
==========================================
- Coverage   91.79%   91.78%   -0.01%     
==========================================
  Files          82       82              
  Lines       12712    12716       +4     
==========================================
+ Hits        11669    11672       +3     
- Misses       1043     1044       +1     

Impacted Files	Coverage Δ
unittests/fixtures.py	81.48% <100.00%> (+0.34%)	⬆️
unittests/test_argparser.py	100.00% <100.00%> (ø)
reframe/core/pipeline.py	92.71% <0.00%> (-0.19%)	⬇️

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[vkarak]

The only change in this PR is that I've wrapped the runtime modifications in a with rt.temp_runtime(fixtures.BUILTIN_CONFIG_FILE): that creates a temporary runtime, so that these unit tests do not mess with the default runtime. The diff is larger due to the new indentation...