Unit tests for ParCSRMatrix overlap code

[victorapm]

• Refactor test_csr_overlap.c and reduce its size
• Add benchmark-matmat mode to test_csr_overlap.c to serve as a reference for ParCSRMatrix overlap calculations
• Add TEST_unit folder in src/test for unit testing
• Add unit test job to machine-tux408.sh regression test

Depends on #1448

[rfalgout]

Looks good. I don't understand the need for the cmake targets, but that's okay. Thanks!

[victorapm]

Thanks, Rob! Just removed an old ctest entry that is not needed. The other two remaining are for 1. running the unit tests when invoking ctest from the build directory and 2. a convenience target that allows running unit tests via make unit_tests