Add debugging info

Author: schlessera

features/db-query.feature

@@ -96,7 +96,7 @@ Feature: Query the database with WordPress' MySQL config
   Scenario: SQL modes do not include any of the modes incompatible with WordPress
     Given a WP install
 
-    When I run `wp db query 'SELECT @@SESSION.sql_mode;'`
+    When I try `wp db query 'SELECT @@SESSION.sql_mode;' --debug`
     Then STDOUT should not contain:
       """
       NO_ZERO_DATE

src/DB_Command.php

@@ -1807,6 +1807,14 @@ protected function get_sql_mode_query( $assoc_args, $modes = [] ) {
 			return '';
 		}
 
+		WP_CLI::debug(
+			sprintf(
+				'SQL mode adaptation is needed: %s => %s',
+				$this->get_current_sql_modes( $assoc_args ),
+				$modes
+			)
+		);
+
 		$modes_str = implode( ',', $modes );
 
 		return "SET SESSION sql_mode='{$modes_str}';";