Add Junit XML output for Python tests.

Verified that this is the correct directory in Jenkins by peeking
over wfarner's shoulder.

Attempted to add coverage as well but it looks like pants doesn't
support anything beyond console output yet (even though pycov supports
coberatura-style output).

Testing Done:
./build-support/jenkins/build.sh

Reviewed at https://reviews.apache.org/r/27098/