Clean up generated files correctly

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D1422

tests/Makefile.am

@@ -33,4 +33,6 @@ newport_client_SOURCES = newport-client.c
 
 EXTRA_DIST= $(service_file).in
 
-MAINTAINERCLEANFILES = $(service_file)
+CLEANFILES = \
+	$(newporttestdata_DATA) \
+	$(NULL)