Improve documentation for running unit tests

The documentation on [Contributing](https://opencolorio.readthedocs.io/en/latest/guides/contributing/contributing.html) 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`