selftests: futex: copy sub-dir test scripts for make O=dir run

For make O=dir run_tests to work, test scripts from sub-directories
need to be copied over to the object directory. Running tests from the
object directory is necessary to avoid making the source tree dirty.

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>