Allow parallel test coverage (e.g. ctest -j 8 )

[nrnhines]

ctest on my machine takes 5.7 minutes Parallel testing with, e.g. ctest -j 8 takes less that 1.5 minutes. (it would be faster but a few of the test times are 69, 64, 40s). Anyway, this PR allows use of parallel ctest when coverage is enabled.

Example:

cmake .. -G Ninja -DCMAKE_INSTALL_PREFIX=install -DCMAKE_BUILD_TYPE=Debug -DNRN_ENABLE_TESTS=ON -DNRN_ENABLE_PERFORMANCE_TESTS=OFF -DNRN_ENABLE_COVERAGE=ON -DNRN_COVERAGE_FILES="src/nrncvode/cvodeobj.cpp;src/nrncvode/netcvode.cpp;src/nrncvode/nrndaspk.cpp;src/nrnmpi/mpispike.cpp"

ninja install
ninja cover_begin
ctest -j 8
ninja cover_html

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[alkino]

Great addition, I didn't know this flag.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.17%. Comparing base (d52f6ae) to head (d5cef02).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3162      +/-   ##
==========================================
- Coverage   67.17%   67.17%   -0.01%     
==========================================
  Files         573      573              
  Lines      111436   111436              
==========================================
- Hits        74855    74854       -1     
- Misses      36581    36582       +1     

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[azure-pipelines]

✔️ d5cef02 -> Azure artifacts URL