testing: expose testing.TB in DBTest instead of full *testing.T (#1500)

Reduce the methods exposed by DBTest to the subset of testing.T exposed
in the testing.TB interface.

driver_test.go

@@ -92,7 +92,7 @@ func init() {
 }
 
 type DBTest struct {
-	*testing.T
+	testing.TB
 	db *sql.DB
 }