Use GNU parallel in Windows CI job

[tautschnig]

This reduces test execution time from 43 minutes down to 15 minutes.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.15%. Comparing base (a8b8f0f) to head (7b4da6c).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8345   +/-   ##
========================================
  Coverage    78.15%   78.15%           
========================================
  Files         1726     1726           
  Lines       188862   188862           
  Branches     18458    18403   -55     
========================================
  Hits        147613   147613           
  Misses       41249    41249           

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