net: fix flaky *TimeoutMustNotReturn tests

net: fix flaky *TimeoutMustNotReturn tests

The tester goroutine doesn't always gets a chance to run before the
timeout expires. Wait for the goroutine to start and set deadlines
before staring the timer.

Fixes #36796

Change-Id: Iffed6259de31340c3f66e34da473826a1d09fcde
Reviewed-on: https://go-review.googlesource.com/c/go/+/412858
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>