[bugfix] Define the set of run environments correctly when -p option is used with regular expressions

[vkarak]

This PR fixes the behaviour of the -p option when it is used with regular expressions. The -p option has also the effect of restricting the programming environments to run with. Although this still works fine in old-style cases, e.g., -p PrgEnv-gnu, it fails to do so when passing regular expressions, for example, -p PrgEnv-(gnu|pgi). Although this will select all the tests that support either PrgEnv-gnu or PrgEnv-pgi, it will not run them. This PR fixes this.

[vkarak]

@jenkins-cscs retry kesch

[codecov-io]

Codecov Report

Merging #716 into master will decrease coverage by <.01%.
The diff coverage is 60%.

@@            Coverage Diff             @@
##           master     #716      +/-   ##
==========================================
- Coverage   91.77%   91.77%   -0.01%     
==========================================
  Files          77       77              
  Lines        9501     9504       +3     
==========================================
+ Hits         8720     8722       +2     
- Misses        781      782       +1

Impacted Files	Coverage Δ
unittests/test_cli.py	96.1% <0%> (-0.32%)	⬇️
reframe/frontend/cli.py	80.79% <100%> (+0.12%)	⬆️

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