storage/db/dbtest: fix -cloud after refactoring

Change-Id: I7bd1ed03127830d6530415d77263ee8b0155a2c4
Reviewed-on: https://go-review.googlesource.com/37230
Run-TryBot: Quentin Smith <quentin@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

storage/db/dbtest/cloud.go

@@ -46,7 +46,7 @@ func createEmptyDB(t *testing.T) (driver, dsn string, cleanup func()) {
 
 	t.Logf("Using database %q", name)
 
-	return prefix + name + "?interpolateParams=true", func() {
+	return "mysql", prefix + name + "?interpolateParams=true", func() {
 		if _, err := db.Exec(fmt.Sprintf("DROP DATABASE `%s`", name)); err != nil {
 			t.Error(err)
 		}