add OpenMP condition to serialize runtests on requests

[jvdp1]

@awvwgk : related to #10

Proposition to avoid running tests in parallel: addition of an optional intent(in) argument parallel in run_testsuite.
If parallel = .true., same behavior as previouly.
If parallel = .false., all tests are run sequentially.

[awvwgk]

Thanks. Looks good to me.

[codecov-commenter]

Codecov Report

Merging #11 (5226fd3) into main (9c3401e) will decrease coverage by 0.07%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   69.10%   69.02%   -0.08%     
==========================================
  Files           2        2              
  Lines         479      481       +2     
  Branches      260      261       +1     
==========================================
+ Hits          331      332       +1     
  Misses         23       23              
- Partials      125      126       +1     

Impacted Files	Coverage Δ
src/testdrive.F90	70.48% <66.66%> (-0.09%)	⬇️

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[jvdp1]

Could this PR be merged?