net: use t.Deadline instead of an arbitrary read deadline in TestDialParallelSpuriousConnection

Also increase the default deadline to 5s, since it empirically
doesn't need to be short and 1s seems to be too slow on some platforms.

Fixes #37795

Change-Id: Ie6bf3916b107401235a1fa8cb0f22c4a98eb2dae
Reviewed-on: https://go-review.googlesource.com/c/go/+/222959


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>