Adds a function for easily accessing the x509.Certificate
of a Server, if there is one. Also adds a helper function
for getting a http.Client suitable for use with the server.

This makes the steps required to test a httptest
TLS server simpler.

Fixes #18411

Change-Id: I2e78fe1e54e31bed9c641be2d9a099f698c7bbde
Reviewed-on: https://go-review.googlesource.com/34639Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>