[bugfix] Treat properly Slurm constraints with .

[Guy-Coates]

The reframe slurm node constraint validator rejects features with a "." in the name (eg nodes with Feature=c5.xlarge defined in slurm).

This results in Reframe generating no tests when attempting to target nodes of that type using --constraint=c5.xlarge in the systems.partitions.access config.

This PR relaxes the validation/tokenisation regexes to allow ".".

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.25%. Comparing base (1aca15b) to head (f824600).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3667   +/-   ##
=======================================
  Coverage   91.25%   91.25%           
=======================================
  Files          62       62           
  Lines       13591    13591           
=======================================
  Hits        12402    12402           
  Misses       1189     1189           

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[vkarak]

Thanks for the fix! Lgtm. Since we are not going to make another patch release for 4.9 (4.10 is around the corner), we'd need to rebase this PR on develop.