We've broken periodic GC a few times without noticing because there's
no test for it, partly because you have to wait two minutes to see if
it happens. This exposes control of the periodic GC timeout to runtime
tests and adds a test that cranks it down to zero and sleeps for a bit
to make sure periodic GCs happen.

Change-Id: I3ec44e967e99f4eda752f85c329eebd18b87709e
Reviewed-on: https://go-review.googlesource.com/13169Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>