Include cupsfilters/testfilters.sh in release tarball

"check_SCRIPTS" is not automatically included, has to be added to
EXTRA_DIST.

Makefile.am

@@ -330,6 +330,7 @@ testfilters_LDFLAGS = \
 	-L/usr/lib
 
 EXTRA_DIST += \
+	$(check_SCRIPTS) \
 	$(pkgfiltersinclude_DATA) \
 	cupsfilters/image.pgm \
 	cupsfilters/image.ppm \
@@ -372,4 +373,4 @@ install-data-hook:
 uninstall-hook:
 	$(RM) $(DESTDIR)$(pkgcharsetdir)/pdf.utf-8
 
-SUBDIRS =
+SUBDIRS =