tests: implement get_test_name()

tests: implement get_test_name()

Screenshot tests often want to use the test name for writing out images.
This is a helper to get the test name without writing it multiple times
in the source.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emilio Pozuelo Monfort <emilio.pozuelo@collabora.co.uk>
Reviewed-by: Micah Fedke <micah.fedke@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>