net/http: reduce TestClientTimeout_h{1,2} latency

The test had been setting an arbitrary 200ms timeout to allow the
server's handler to set up before timing out. That is not only
potentially flaky on slow machines, but also typically much longer
than necessary. Replace the hard-coded timeout with a much shorter
initial timeout, and use exponential backoff to lengthen it if needed.

This allows the test to be run about 20x faster in the typical case,
which may make it easier to reproduce rare failure modes by running
with a higher -count flag.

For #43120

Change-Id: I1e0d0ec99d5a107fff56e3bcc7174d686ec582d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/361275


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>