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.

framework/core/tests/phpunit.integration.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="UTF-8"?>
+
+<phpunit backupGlobals="false"
+         backupStaticAttributes="false"
+         colors="true"
+         convertErrorsToExceptions="true"
+         convertNoticesToExceptions="true"
+         convertWarningsToExceptions="true"
+         processIsolation="false"
+         stopOnFailure="false"
+         syntaxCheck="false">
+
+    <testsuites>
+        <testsuite name="Flarum Integration Tests">
+            <directory suffix="Test.php">./integration</directory>
+        </testsuite>
+    </testsuites>
+    <filter>
+        <whitelist processUncoveredFilesFromWhitelist="true">
+            <directory suffix=".php">./src/</directory>
+        </whitelist>
+    </filter>
+</phpunit>