database/sql: fix possible context leak in test

Fixes #17560

Change-Id: I96fcdec87220391ef5432571b5c090b5be27491a
Reviewed-on: https://go-review.googlesource.com/31771Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql_test.go

@@ -267,6 +267,7 @@ func TestQueryContext(t *testing.T) {
 	prepares0 := numPrepares(t, db)
 
 	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 
 	rows, err := db.QueryContext(ctx, "SELECT|people|age,name|")
 	if err != nil {