selftests/mm: Optionally pass duration to transhuge-stress

Until now, transhuge-stress runs until its explicitly killed, so when
invoked by run_kselftest.sh, it would run until the test timeout, then
it would be killed and the test would be marked as failed.

Add a new, optional command line parameter that allows the user to
specify the duration in seconds that the program should run. The program
exits after this duration with a success (0) exit code. If the argument
is omitted the old behacvior remains.

On it's own, this doesn't quite solve our problem because
run_kselftest.sh does not allow passing parameters to the program under
test. But we will shortly move this to run_vmtests.sh, which does allow
parameter passing.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>