net/http: add a benchmark for multi indexing

net/http: add a benchmark for multi indexing

We don't index multis, so a corpus full of them will take quadratic
time to check for conflicts. How slow is that going to be in practice?

This benchmark indexes and checks a thousand multi patterns, all disjoint.
It runs in about 35ms.

Change-Id: Id27940ab19ad003627bd5c43c53466e01456b796
Reviewed-on: https://go-review.googlesource.com/c/go/+/529477
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>