Use MySQL 5.6 for WP 4.9 tests

Follow-up to 9915e45 / #121. MySQL 8 support wasn't added until later.

Author: swissspidy

.github/workflows/reusable-testing.yml

@@ -46,7 +46,7 @@ jobs:
             {
               "php": "7.2",
               "wp": "4.9",
-              "mysql": "8.0"
+              "mysql": "5.6"
             },
             {
               "php": "7.2",