Test against dev version of core

Since we're making changes for the dev version of core, we should test against that too.

For now, this fixes issues with the `extensions` method of `Installation` being undefined.

php-packages/testing/composer.json

@@ -13,7 +13,7 @@
         "phpunit/phpunit": "^9.0"
     },
     "require-dev": {
-        "flarum/core": "^0.1.0-beta"
+        "flarum/core": "*@dev"
     },
     "autoload": {
         "psr-4": {

php-packages/testing/tests/composer.json

@@ -3,7 +3,7 @@
   "description": "Minimal extension to test the flarum/testing package",
   "type": "flarum-extension",
   "require": {
-    "flarum/core": "^0.1.0-beta.16"
+    "flarum/core": "^0.1.0@dev"
   },
   "require-dev": {
     "flarum/testing": "*@dev"