Revert Testbench upgrade - upgrade requires upgrading PHPUnit, which has breaking changes

composer.json

@@ -26,7 +26,7 @@
         "league/flysystem": "^1.0"
     },
     "require-dev": {
-        "orchestra/testbench": "^3.5|^4.0",
+        "orchestra/testbench": "3.5.* || 3.6.* || 3.7.*",
         "phpunit/phpunit": "^6.0.0 || ^7.4.0",
         "dingo/api": "2.0.0-alpha1",
         "mockery/mockery": "^1.2.0",