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.

tests/integration/TestCase.php

@@ -11,21 +11,71 @@
 
 namespace Flarum\Tests\integration;
 
-use PHPUnit\Framework\TestCase as Test;
+use Flarum\Foundation\InstalledSite;
+use Illuminate\Database\ConnectionInterface;
 
-abstract class TestCase extends Test
+abstract class TestCase extends \PHPUnit\Framework\TestCase
 {
-    use CreatesForum,
-        MakesApiRequests;
-
     public function setUp()
     {
-        $this->refreshApplication();
-        $this->init();
+        parent::setUp();
+
+        // Boot the Flarum app
+        $this->app();
     }
 
-    protected function init()
+    protected $app;
+
+    /**
+     * @return \Flarum\Foundation\InstalledApp
+     */
+    protected function app()
     {
-        // .. allows implementation by children without the need to call the parent.
+        if (! is_null($this->app)) {
+            return $this->app;
+        }
+
+        $site = new InstalledSite(
+            [
+                'base' => __DIR__.'/tmp',
+                'public' => __DIR__.'/tmp/public',
+                'storage' => __DIR__.'/tmp/storage',
+            ],
+            include __DIR__.'/tmp/config.php'
+        );
+
+        return $this->app = $site->bootApp();
+    }
+
+    protected $database;
+
+    protected function database(): ConnectionInterface
+    {
+        if (is_null($this->database)) {
+            $this->database = $this->app()->getContainer()->make(
+                ConnectionInterface::class
+            );
+        }
+
+        return $this->database;
+    }
+
+    protected function prepareDatabase(array $tableData)
+    {
+        // We temporarily disable foreign key checks to simplify this process.
+        $this->database()->getSchemaBuilder()->disableForeignKeyConstraints();
+
+        // First, truncate all referenced tables so that they are empty.
+        foreach (array_keys($tableData) as $table) {
+            $this->database()->table($table)->truncate();
+        }
+
+        // Then, insert all rows required for this test case.
+        foreach ($tableData as $table => $rows) {
+            $this->database()->table($table)->insert($rows);
+        }
+
+        // And finally, turn on foreign key checks again.
+        $this->database()->getSchemaBuilder()->enableForeignKeyConstraints();
     }
 }