selftests/futex: Update Makefile to use lib.mk

Adapt the futextest Makefiles to use lib.mk macros for RUN_TESTS and
EMIT_TESTS. For now, we reuse the run.sh mechanism provided by
futextest. This doesn't provide the standard selftests: [PASS|FAIL]
format, but the tests provide very similar output already.

This results in the run_kselftest.sh script for futexes including a
single line: ./run.sh

Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: linux-api@vger.kernel.org
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>