Issue #1706878 by tim.plunkett, lazysoundsystem: Added DrupalWebTestCase::assertThemeOutput() to allow modules to test theme function output.

CHANGELOG.txt

@@ -1,6 +1,8 @@
 
 Drupal 7.18, xxxx-xx-xx (development version)
 -----------------------
+- Added an assertThemeOutput() method to Simpletest to allow tests to check
+  that themed output matches an expected HTML string (API addition).
 - Added a link to "Install another module" after a module has been successfully
   downloaded via the Update Manager (UI change).
 - Added an optional "exclusive" flag to installation profile .info files which

modules/simpletest/drupal_web_test_case.php

@@ -3148,6 +3148,42 @@ class DrupalWebTestCase extends DrupalTestCase {
     return $this->assertNotEqual($actual, $title, $message, $group);
   }
 
+  /**
+   * Asserts themed output.
+   *
+   * @param $callback
+   *   The name of the theme function to invoke; e.g. 'links' for theme_links().
+   * @param $variables
+   *   An array of variables to pass to the theme function.
+   * @param $expected
+   *   The expected themed output string.
+   * @param $message
+   *   (optional) A message to display with the assertion. Do not translate
+   *   messages: use format_string() to embed variables in the message text, not
+   *   t(). If left blank, a default message will be displayed.
+   * @param $group
+   *   (optional) The group this message is in, which is displayed in a column
+   *   in test output. Use 'Debug' to indicate this is debugging output. Do not
+   *   translate this string. Defaults to 'Other'; most tests do not override
+   *   this default.
+   *
+   * @return
+   *   TRUE on pass, FALSE on fail.
+   */
+  protected function assertThemeOutput($callback, array $variables = array(), $expected, $message = '', $group = 'Other') {
+    $output = theme($callback, $variables);
+    $this->verbose('Variables:' . '<pre>' .  check_plain(var_export($variables, TRUE)) . '</pre>'
+      . '<hr />' . 'Result:' . '<pre>' .  check_plain(var_export($output, TRUE)) . '</pre>'
+      . '<hr />' . 'Expected:' . '<pre>' .  check_plain(var_export($expected, TRUE)) . '</pre>'
+      . '<hr />' . $output
+    );
+    if (!$message) {
+      $message = '%callback rendered correctly.';
+    }
+    $message = format_string($message, array('%callback' => 'theme_' . $callback . '()'));
+    return $this->assertIdentical($output, $expected, $message, $group);
+  }
+
   /**
    * Asserts that a field exists in the current page by the given XPath.
    *