test-runner: wait for concrete pid

After running a test in fork, we were waiting for any child to terminate.
It is OK unless the child forks again. If the child calls fork, the waitid can
catch the child's child termination, stop block and run another test
while the former test is still running. This is racy i. e. when adding socket.
Since we have test compositor which uses fork, this situation can occur
pretty frequently.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>