Use greenmail IMAP server for testing message rendering

It's already integrated in the test-suite and the upstream image runs on
arm64, too.

.ci/compose.yaml

@@ -1,6 +1,10 @@
 services:
   mailhost:
     image: docker.io/greenmail/standalone
+    environment:
+      GREENMAIL_OPTS: '-Dgreenmail.setup.test.all -Dgreenmail.hostname=0.0.0.0 -Dgreenmail.auth.disabled -Dgreenmail.preload.dir=/home/greenmail/preload'
+    volumes:
+      - "../tests/MessageRendering/data/greenmail:/home/greenmail/preload"
   browserhost:
     image: docker.io/selenium/standalone-chromium
     ports:
@@ -40,6 +44,17 @@ services:
     command:
       - .ci/run_tests.sh
 
+  test_message_rendering:
+    depends_on:
+      - mailhost
+    image: ghcr.io/roundcube/roundcubemail-testrunner:php8.3
+    environment:
+      RC_CONFIG_IMAP_HOST: 'tls://mailhost:3143'
+    volumes:
+      - '..:/app'
+    command:
+      - .ci/run_test_message_rendering.sh
+
   codespell:
     image: ghcr.io/roundcube/roundcubemail-testrunner:php8.3
     volumes:

.ci/run_test_message_rendering.sh

@@ -0,0 +1,11 @@
+#!/bin/bash -ex
+
+if ! test -f config/config-test.inc.php; then
+	cp -v .ci/config-test.inc.php config/config-test.inc.php
+fi
+
+# Install dependencies, prefer highest.
+composer update --prefer-dist --no-interaction --no-progress
+
+# Execute tests.
+vendor/bin/phpunit -c ./tests/MessageRendering/phpunit.xml --fail-on-warning --fail-on-risky

tests/MessageRendering/MessageRenderingTestCase.php

@@ -3,11 +3,13 @@
 namespace Tests\MessageRendering;
 
 use Masterminds\HTML5;
+use Roundcube\Tests\ActionTestCase;
+use Roundcube\Tests\ExitException;
 
 /**
  * Class to base actual test classes on, which test specific message rendering.
  */
-class MessageRenderingTestCase extends \ActionTestCase
+class MessageRenderingTestCase extends ActionTestCase
 {
     /**
      * Get the body from the document, trimmed from surrounding whitespace.
@@ -40,13 +42,14 @@ protected function getScrubbedSubject(\DOMXPath $domxpath): string
      */
     protected function runAndGetHtmlOutputDomxpath(string $msgId): \DOMXPath
     {
+        $imap_host = getenv('RC_CONFIG_IMAP_HOST') ?: 'tls://localhost:143';
         $rcmail = \rcmail::get_instance();
         // We need to overwrite the storage object, else storage_init() just
         // returns the cached one (which might be a StorageMock instance).
         $mockStorage = $rcmail->storage = null;
         $rcmail->storage_init();
         // Login our test user so we can fetch messages from the imap server.
-        $rcmail->login('test', 'pass', 'tls://localhost');
+        $rcmail->login('test-message-rendering@localhost', 'pass', $imap_host);
         $storage = $rcmail->get_storage();
         $storage->set_options(['all_headers' => true]);
         // We need to set the folder, else no message can be fetched.
@@ -77,7 +80,7 @@ protected function runAndGetHtmlOutputDomxpath(string $msgId): \DOMXPath
         $html = '';
         try {
             $action->run();
-        } catch (\ExitException $e) {
+        } catch (ExitException $e) {
             $html = $output->getOutput();
         }