Hook up the Python-based unit tests for NVScoreVariants to our test suite

[droazen]

There are a number of Python-based unit tests for NVScoreVariants in src/main/python/org/broadinstitute/hellbender/scorevariants/tests, which use the standard Python unittest framework (https://docs.python.org/3/library/unittest.html).

We should ideally hook these up to the GATK test suite, and run them either via a gradle plugin for Python tests, or via a PythonScriptExecutor from a Java-based TestNG test. We'll need to figure out how to parse the test report for these tests and get the results to display nicely in github alongside the Java-based test results. We'll also need to look into whether the tests are cleaning up temp files properly, etc.