This deprecates PYTHON_UNIT_TESTS and replaces it with PYTHON_UNITTEST_MANIFESTS.
In the build system, this means python unittests will be treated the same as all
other test suites that use manifestparser. New manifests called 'python.ini' have
been created for all test directories containing python unittests.
MozReview-Commit-ID: IBHG7Thif2D
--HG--
extra : rebase_source : 11a92a2bc544d067946bbd774975140147458caa