selftests: futex: fix run_tests target

make -C tools/testing/selftests/futex/ run_tests doesn't run the futex
tests.

Running the tests when `dirname $(OUTPUT)` == $(PWD) doesn't work when
the $(OUTPUT) is $(PWD) which is the case when the test is run using
make -C tools/testing/selftests/futex/ run_tests.

Fixes: a8ba798b ("selftests: enable O and KBUILD_OUTPUT")
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Reviewed-by: Darren Hart (VMware) <dvhart@infradead.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>