net: Set finalDeadline from TestDialParallel to avoid leaked sockets.

I've also changed TestDialSerialAsyncSpuriousConnection for consistency,
although it always computes a finalDeadline of zero.

Note that #11225 is the root cause of the socket leak; this just hides
it from the unit test by restoring the shorter timeout.

Fixes #11878

Change-Id: Ie0037dd3bce6cc81d196765375489f8c61be74c2
Reviewed-on: https://go-review.googlesource.com/12712


Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Paul Marks <pmarks@google.com>