acme/autocert: make tests more well-behaved

This change also gets the Manager closer to being able
to cleanup in short-lived HTTP servers running in a long-lived binary.

Change-Id: I49db36156896acc76d4757146c26b99e1665423b
Reviewed-on: https://go-review.googlesource.com/28491
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>