Issue #3471949 by mstrelan: Allow passing MarkupInterface to AssertContentTrait::setRawContent

(cherry picked from commit 76617730)

core/tests/Drupal/KernelTests/AssertContentTrait.php

@@ -56,7 +56,7 @@ protected function getRawContent() {
   /**
    * Sets the raw content (e.g. HTML).
    *
-   * @param string $content
+   * @param string|\Stringable $content
    *   The raw content to set.
    */
   protected function setRawContent($content) {