tests: Rename tests/Makefile to tests/Makefile.include

The file is only included from the top Makefile. Rename it to reflect
this more obviously.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <1464747811-26917-1-git-send-email-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>