selftests: mqueue: Use full path to run tests from Makefile

Use full path including $(OUTPUT) to run tests from Makefile for
normal case when objects reside in the source tree as well as when
objects are relocated with make O=dir. In both cases $(OUTPUT) will
be set correctly by lib.mk.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>