net: retry TestDialTimeout subtests with progressively shorter timeouts

net: retry TestDialTimeout subtests with progressively shorter timeouts

The LUCI builders seem to show that the failure mode in #62359 is not
specific to windows/arm64, but it occurs to me that we ought to be
able to eventually retry by making the timeout so short that the
remote end can't possibly respond in time (discounting the possibility
that the kernel itself might short-circuit the loopback address).

For #62377.
Updates #62359.
Updates #56876.

Change-Id: I1fb5fa4f2a5d2cfe35465f34248ed9a035f91f4f
Reviewed-on: https://go-review.googlesource.com/c/go/+/524595
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>