Advertise the new PythonTest_pytests addition.

For those who use the 'make help' paradigm to look for an entry point to
how to run their tests.

Please see this commit for context:
  4e887567c5b4b06646ab1340376e240d6c5af9cb

Change-Id: Id7ffed3aa1e5157406c0c3d5328f4dd5b7d81178
Reviewed-on: https://gerrit.libreoffice.org/8155
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>