First working test.

tests/phpunit/ParseOnEditTest.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * @group bovender
+ * @group Database
+ */
+class ParseOnEditTest extends LinkTitles\TestCase {
+
+  public function testParseOnEdit() {
+    $this->setMwGlobals( [
+      'wgLinkTitlesParseOnEdit' => true,
+      'wgLinkTitlesParseOnRender' => true
+    ] );
+    $pageId = $this->insertPage( 'test page', 'This page should link to the link target' )['id'];
+    $page = WikiPage::newFromId( $pageId );
+    $this->assertSame( 'This page should link to the [[link target]]', self::getPageText( $page ) );
+  }
+}

tests/phpunit/TestCase.php

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+<?php
+namespace LinkTitles;
+
+abstract class TestCase extends \MediaWikiTestCase {
+  protected function setUp() {
+    parent::setUp();
+    $this->insertPage( 'link target', 'This page serves as a link target' );
+    Extension::invalidateCache();
+  }
+
+  protected function tearDown() {
+    parent::tearDown();
+  }
+
+  protected function getPageText( \WikiPage $page ) {
+    $content = $page->getContent();
+    return $page->getContentHandler()->serializeContent( $content );
+  }
+}