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>

src/database/sql/sql_test.go

@@ -2841,7 +2841,6 @@ func TestTxStmtDeadlock(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 	tx, err := db.BeginTx(ctx, nil)
-	cancel()
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -2850,6 +2849,7 @@ func TestTxStmtDeadlock(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	cancel()
 	// Run number of stmt queries to reproduce deadlock from context cancel
 	for i := 0; i < 1e3; i++ {
 		// Encounter any close related errors (e.g. ErrTxDone, stmt is closed)