[bugfix] Add a persistent version of rmtree()

[vkarak]

Fixes #291.

[codecov-io]

Codecov Report

Merging #437 into master will increase coverage by 0.01%.
The diff coverage is 95.12%.

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
+ Coverage   91.21%   91.23%   +0.01%     
==========================================
  Files          70       70              
  Lines        8585     8611      +26     
==========================================
+ Hits         7831     7856      +25     
- Misses        754      755       +1

Impacted Files	Coverage Δ
unittests/test_utility.py	98.81% <100%> (+0.04%)	⬆️
reframe/core/pipeline.py	92.48% <100%> (ø)	⬆️
reframe/core/runtime.py	87.28% <100%> (-0.11%)	⬇️
unittests/test_schedulers.py	97.72% <100%> (-0.01%)	⬇️
unittests/test_pipeline.py	97.37% <100%> (ø)	⬆️
unittests/test_policies.py	99.09% <100%> (-0.46%)	⬇️
unittests/test_cli.py	95.89% <100%> (-0.02%)	⬇️
reframe/utility/os_ext.py	89.65% <86.66%> (-0.35%)	⬇️
reframe/core/config.py	84.54% <0%> (+1.81%)	⬆️

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