Update mantle-framework/testkit requirement from ^0.7 to ^0.9

Updates the requirements on [mantle-framework/testkit](https://github.com/mantle-framework/testkit) to permit the latest version.
- [Commits](mantle-framework/testkit@v0.7.0...v0.9.1)

---
updated-dependencies:
- dependency-name: mantle-framework/testkit
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <[email protected]>

composer.json

@@ -23,7 +23,7 @@
     "require-dev": {
         "alleyinteractive/alley-coding-standards": "^2.0",
         "alleyinteractive/composer-wordpress-autoloader": "^1.0",
-        "mantle-framework/testkit": "^0.7",
+        "mantle-framework/testkit": "^0.9",
         "nunomaduro/collision": "^5.0"
     },
     "config": {