Add PHP 7.2 / WP 4.9 to test matrix

WP 4.9 is the oldest version with PHP 7.2 compatibility

Author: swissspidy

.github/workflows/reusable-testing.yml

@@ -43,6 +43,11 @@ jobs:
               "wp": "6.2",
               "mysql": "8.0"
             },
+            {
+              "php": "7.2",
+              "wp": "4.9",
+              "mysql": "8.0"
+            },
             {
               "php": "7.2",
               "wp": "latest",