acme/autocert: improve test speed on 386

This change makes the tests run considerably faster on GOARCH=386
by reducing test RSA keys to 512-bit size.

It also increases GetCertificate test timeout to allow for slower
computations.

Change-Id: I7a27ffa5130f9ba08ee2069dad29aed74fa8c521
Reviewed-on: https://go-review.googlesource.com/27094
Run-TryBot: Alex Vaghin <ddos@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>