[bugfix] Avoid conflicts with already loaded Python modules when loading ReFrame tests

[rsarm]

Closes #2336

[codecov-commenter]

Codecov Report

Merging #2390 (b487095) into master (f02136b) will increase coverage by 0.27%.
The diff coverage is 100.00%.

❗ Current head b487095 differs from pull request most recent head 1c1c773. Consider uploading reports for the commit 1c1c773 to get more accurate results

@@            Coverage Diff             @@
##           master    #2390      +/-   ##
==========================================
+ Coverage   85.86%   86.13%   +0.27%     
==========================================
  Files          56       56              
  Lines       10270    10228      -42     
==========================================
- Hits         8818     8810       -8     
+ Misses       1452     1418      -34     

Impacted Files	Coverage Δ
reframe/utility/__init__.py	92.64% <100.00%> (-0.12%)	⬇️
reframe/frontend/cli.py	76.42% <0.00%> (-0.09%)	⬇️
reframe/core/pipeline.py	92.83% <0.00%> (-0.06%)	⬇️
reframe/frontend/executors/__init__.py	98.29% <0.00%> (+0.78%)	⬆️
reframe/core/systems.py	89.47% <0.00%> (+1.31%)	⬆️
reframe/core/schedulers/local.py	96.63% <0.00%> (+3.36%)	⬆️
reframe/frontend/executors/policies.py	99.69% <0.00%> (+6.90%)	⬆️

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[vkarak]

Lgtm. Just a couple of minor comments still.