[bugfix] Fix regression with tests that treat stdin as a tty

[teojgo]

Fixes #2466

[teojgo]

@giordano I could reproduce your issue. Can you check that it's fixed by this?

[giordano]

Yes, I can confirm the reproducer in #2466 works for me with this PR, thanks!

[vkarak]

@teojgo This was regression indeed in ReFrame and, in fact, we used to set stdin to DEVNULL as a fix of #1877 in #1879. This was accidentally removed in #2383. @teojgo could have a unit test to reproduce this? #1877 has a reproducer that's not relying on Spack. We could add a unit test for it.

[codecov-commenter]

Codecov Report

Merging #2476 (a053a30) into master (f44e37c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head a053a30 differs from pull request most recent head 9793e74. Consider uploading reports for the commit 9793e74 to get more accurate results

@@           Coverage Diff           @@
##           master    #2476   +/-   ##
=======================================
  Coverage   85.70%   85.70%           
=======================================
  Files          57       57           
  Lines       10544    10545    +1     
=======================================
+ Hits         9037     9038    +1     
  Misses       1507     1507           

Impacted Files	Coverage Δ
reframe/utility/osext.py	84.42% <100.00%> (+0.06%)	⬆️

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