net/http: rename tests for Redirect and StripPrefix

Before May 2018, I mistakenly thought the _suffix naming convention¹
used by examples also applied to tests. Thanks to a code review comment²
from Ian Lance Taylor, I have since learned that is not true.
This trivial change fixes some collateral damage from my earlier
misunderstanding, resulting in improved test naming consistency.

¹ https://golang.org/pkg/testing/#hdr-Examples
² https://go-review.googlesource.com/c/go/+/112935/1/src/path/filepath/path_test.go#1075

Change-Id: I555f60719629eb64bf2f096aa3dd5e00851827cd
Reviewed-on: https://go-review.googlesource.com/c/go/+/207446


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>