Improve documentation for running unit tests

[doug-walker]

The documentation on Contributing has some basic information about running the unit tests. However, it would be helpful to extend that to cover situations such as:

• Running individual tests rather than all tests
• Running only the CPU, GPU, or Python tests rather than all tests

For example, this will run only the CPU unit tests from the Builtins suite: tests/cpu/test_cpu_exec --run_only Builtins

And this will run just Python test: python3 ../tests/python/OpenColorIOTestSuite.py $PWD

[JaydenShel]

Can I take this?

[remia]

To just run one category of unit test, it might be simpler to stick with ctest:

# Get the list of tests
ctest -N
# Run individual tests examples
ctest -V -R test_cpu
ctest -V -R test_gpu
ctest -V -R test_python

But to reach individual tests within a group, I think we have to use our executable directly with --run_only like Doug showed.