[bugfix] Fix unit test failure for scheduler backends when run alone

[vkarak]

Fixes #1075.

[codecov-io]

Codecov Report

Merging #1076 into master will increase coverage by <.01%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #1076      +/-   ##
==========================================
+ Coverage   91.66%   91.67%   +<.01%     
==========================================
  Files          81       81              
  Lines       11078    11089      +11     
==========================================
+ Hits        10155    10166      +11     
  Misses        923      923

Impacted Files	Coverage Δ
unittests/test_cli.py	93.27% <0%> (ø)	⬆️
unittests/test_schedulers.py	96.86% <100%> (+0.01%)	⬆️
reframe/core/runtime.py	88.96% <100%> (+0.6%)	⬆️

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