net: eliminate arbitrary timeout in TestVariousDeadlines

When we set a timeout, we don't actually have a guarantee one how long
the OS will take to notice it. Moreover, if the test deadlocks
completely (for example, due to a deadline never taking effect), it
would be more useful to get a full goroutine dump instead of the current
"client stuck in Dial+Copy" failure message.

For #37883
For #41863

Change-Id: I9f712ef1c620f97a5ab69baac45deb71134b99bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/371994


Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>