tests belong to check target

- the CustomTarget seems to be a test, not a build target
- worse, it is .PHONY
- ... and eats 30% of the top-level build-nocheck time alone
- so hopefully, we can we at least make it a check target

Change-Id: I113777cbd849aab9e8d0a77184e31194bc07232e
Reviewed-on: https://gerrit.libreoffice.org/8111
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>