rc_subr_test: Bump some sleep timeouts

The test verifies that the rc framework will OOM-protect a process
spawned by rc. It just wraps a 5-second /bin/sleep invocation as part
of this test.

The rc framework uses procctl to set the OOM-protect bit after the
process has started, i.e., it uses procctl -p. So, with a 5 second
timeout, it's possible for the process to exit before procctl actually
runs, if the system is heavily loaded. (I see this failure occasionally
with KMSAN configured and many tests running in parallel.)

Bump the timeout to reduce the risk of this happening. The timeout
value is arbitrary since the test will stop the rc process, i.e., we
don't have to wait for 60 seconds to elapse before the test passes.

MFC after: 1 week