screenshot: add screenshot ui test to check target

Change-Id: I6f3540f919f109b88628ed081a184b9e29a35450
Reviewed-on: https://gerrit.libreoffice.org/28203Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

Makefile.in

@@ -7,7 +7,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 #
 
-gb_Top_MODULE_CHECK_TARGETS := slowcheck unitcheck subsequentcheck perfcheck uicheck
+gb_Top_MODULE_CHECK_TARGETS := slowcheck unitcheck subsequentcheck perfcheck uicheck screenshot
 
 .PHONY : all check-if-root bootstrap gbuild build build-non-l10n-only build-l10n-only check clean clean-build clean-host test-install distclean distro-pack-install docs download etags fetch findunusedcode get-submodules id install install-strip tags debugrun help showmodules translations packageinfo internal.clean $(gb_Top_MODULE_CHECK_TARGETS)