[bugfix] Fix unit tests leaving stale files behind

[vkarak]

Fixes #947.

[codecov-io]

Codecov Report

Merging #948 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #948      +/-   ##
==========================================
+ Coverage   91.69%   91.73%   +0.03%     
==========================================
  Files          79       79              
  Lines       10465    10465              
==========================================
+ Hits         9596     9600       +4     
+ Misses        869      865       -4

Impacted Files	Coverage Δ
unittests/test_pipeline.py	96.49% <100%> (+0.41%)	⬆️
reframe/core/config.py	85.71% <0%> (+1.78%)	⬆️

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