Make PythonTests depend on the ExternalExecutable

This fixes builds on windows with fully-internal python
where the package is built but the exe isn't in instdir
when building a PythonTest.

Change-Id: Iab1d330a36adf1c849635b990bfe5d31359bec2b
Reviewed-on: https://gerrit.libreoffice.org/11618
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>