Make integration tests independent

This creates a dedicated test suite for integration tests. All of them
can be run independently, and there is no order dependency - previously,
all integration tests needed the installer test to run first, and they
would fail if installation failed.

Now, the developer will have to set up a Flarum database to be used by
these tests. A setup script to make this simple will be added in the
next commit.

Small tradeoff: the installer is NOT tested in our test suite anymore,
only implicitly through the setup script. If we decide that this is a
problem, we can still set up separate, dedicated installer tests which
should probably test the web installer.

src/Foundation/InstalledApp.php

@@ -42,6 +42,11 @@ class InstalledApp implements AppInterface
         $this->config = $config;
     }
 
+    public function getContainer()
+    {
+        return $this->container;
+    }
+
     /**
      * @return \Psr\Http\Server\RequestHandlerInterface
      */

src/Foundation/InstalledSite.php

@@ -72,7 +72,7 @@ class InstalledSite implements SiteInterface
     /**
      * Create and boot a Flarum application instance.
      *
-     * @return AppInterface
+     * @return InstalledApp
      */
     public function bootApp(): AppInterface
     {