net/http: fix test TestServeMuxHandlerRedirects

net/http: fix test TestServeMuxHandlerRedirects

The code was intended to test that mux handler should redirect at
most once, but the added loop condition defeated that. Remove the
loop condition and document the intention better.

Fixes #18068.

Change-Id: I2a4ea041eae27168b45a09aa46e740ac03921594
Reviewed-on: https://go-review.googlesource.com/33654
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>