net/http: skip TestClientTimeout_h{1,2} on windows/arm and windows/arm64

These tests are empirically flaky on the windows/arm and windows/arm64
builders, with a consistent (but rare) failure mode.

This change skips the test if that particular failure mode is
encountered on those platforms; the skip can be removed if and when
someone has the time to pin down the root cause.

For #43120

Change-Id: Ie3a9a06bf47e3a907c7b07441acc1494a4631135
Reviewed-on: https://go-review.googlesource.com/c/go/+/375635


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