Add support for --test-custom/PYBUILD_TEST_CUSTOM

This supports things like this in debian/rules (after listing runtests.py in debian/pybuild.testfiles):

export PYBUILD_TEST_CUSTOM = 1
export PYBUILD_TEST_ARGS = python{version} runtests.py

This should cover most use cases of using dh_override_auto_test, as one can now specify the full test command to run. The command also is implicitly executed with all desired python versions, since the test running is driven by pybuild. avoiding writing loops by hand in shell inside debian/rules.