signal/Makefile: Split and sort tests

This will make it easier to add tests later; see also the test section
in malloc/Makefile that inspires this.

Signed-off-by: Michel Lind <salimma@fedoraproject.org>
Suggested-by: Arjun Shankar <arjun@redhat.com>
Reviewed-by: Arjun Shankar <arjun@redhat.com>