This was introduced in commit 7846d615

: g_subprocess_get_identifier()
will return NULL after the subprocess has exited, and the subprocess in
the `noop` test will exit as soon as it has started spawning. So if the
scheduler scheduled the testprog subprocess quickly, descheduled the
parent test process until the testprog exited, then the return value
from g_subprocess_get_identifier() would be NULL.

Move the g_subprocess_get_identifier() test to one which calls testprog
in `sleep-forever` mode, since that is guaranteed not to exit until
killed (which we do later in the test).

Signed-off-by: Philip Withnall <withnall@endlessm.com>