In order to keep the kselftest Makefiles simpler, set the threadtest
default values to the ones used in standard run_tests

Cc: Shuah Khan <shuahkh@osg.samsung.com>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>