runtime: remove arbitrary timeouts in finalizer tests
These short timeouts can overrun due to system scheduling delay
(or GC latency) on a slow or heavily-loaded host.
Moreover, if the test deadlocks we will probably want to know what the
GC goroutines were doing at the time. With an arbitrary timeout, we
never get that information; however, if we allow the test to time out
completely we will get a goroutine dump (and, if GOTRACEBACK is
configured in the environment, that may even include GC goroutines).