Add ROOT_ADD_PYUNITTEST and ROOT_ADD_PYUNITTESTS functions to the CMake code
Add ROOT_ADD_PYUNITTEST and ROOT_ADD_PYUNITTESTS functions to the CMake code

in order to make even easier to test python code in source.
The rationale behind the two function is:
- Use ROOT_ADD_PYUNITTESTS(testName) to add all tests in all *py files in the current directory
- Use ROOT_ADD_PYUNITTEST(testName my/path/to/mytests.py) to add all tests in my/path/to/mytests.py

The latter option allows to parallelise the execution of the tests.

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