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>

The time granularity on windows is large enough that setting even an
implausibly small timeout still gives ConnectEx enough time to succeed
before the timeout expires. That causes TestDialTimeout to sometimes
flake, because it expects to be able to provoke a timeout using some
nonzero duration.

This change takes a two-pronged approach to address the problem:

1. We can set a deadline on the FD more aggressively. (If the Context
has already expired, or the deadline is already known, we can go ahead
and set it on the fd without waiting for a background goroutine to get
around to it.)

2. We can reintroduce a test hook to ensure that Dial takes a
measurable amount of time before it completes, so that setting an
implausibly short deadline sets that deadline in the past instead of
the future.

Together, these reduce the flake rate on a windows-amd64-longtest
gomote from around 1-in-10 to less than 1-in-2000.

For #62377.

Change-Id: I03975c32f61fffa9f6f84efb3c474a01ac5a0d1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/524936
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>