database/sql: do not rely on timeout for deadlock test

Fixes #46783

Change-Id: I8a8d1716279a041a7411c0c47a440a7997b39c80
Reviewed-on: https://go-review.googlesource.com/c/go/+/328649


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Carlos Amedee <carlos@golang.org>