[ci] Serialize unit tests everywhere

[vkarak]

For a couple of reasons:

1. pytest-parallel is not yet stable (0.x versions)
2. Running multiple instances of ReFrame at the same time is not supported and can have unpredictable side effects.

[teojgo]

@jenkins-cscs retry daint

[codecov-commenter]

Codecov Report

Merging #2068 (695cbac) into master (4f3d906) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2068   +/-   ##
=======================================
  Coverage   86.76%   86.76%           
=======================================
  Files          52       52           
  Lines        9261     9261           
=======================================
  Hits         8035     8035           
  Misses       1226     1226           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4f3d906...695cbac. Read the comment docs.