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:
  4e887567

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

solenv/gbuild/gbuild.help.txt

@@ -72,6 +72,7 @@ AVAILABLE TARGETS
        o Library
        o Package
        o Pagein
+       o PythonTest_pytests
        o Pyuno
        o Rdb
        o StaticLibrary