net/http: remove more arbitrary timeouts from server tests

This change eliminates the easy, arbitrary timouts that should
never happen. It leaves in place a couple of more complicated ones
that will probably need retry loops for robustness.

For #49336.
For #36179.

Change-Id: I657ef223a66461413a915da5ce9150f49acec04a
Reviewed-on: https://go-review.googlesource.com/c/go/+/476035
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>