database/sql: fix deadlock test in prepare statement

The issue go#46783 correctly diagnosed the context timeout
caused an intermittent failure when the context was canceled
prior to the BeginTx call. However due to the asynchronous nature
of canceling a Tx through a context on fast systems, the tx.Prepare
also succeeded. On slower systems or if a time.Sleep was inserted
between the BeginTx and Prepare, the Prepare would fail.

Resolve this by moving the context cancel after the Prepare.
This will still trigger the deadlock which I tested locally.
In addition, I interspersed multiple time.Sleep calls and the
test still functioned.

Fixes #46852

Change-Id: I9cbf90d3c12b2555493a37799738772b615ae39d
Reviewed-on: https://go-review.googlesource.com/c/go/+/329830


Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>