Remove !placeholder in NodeRevisionUiTest and make it actually test the expected output

Follow-up to #2506445: Replace !placeholder with @placeholder in t() and format_string() for non-URLs in tests

Problem/Motivation

From #2506445-184: Replace !placeholder with @placeholder in t() and format_string() for non-URLs in tests

+++ b/core/modules/node/src/Tests/NodeRevisionsUiTest.php
+++ b/core/modules/node/src/Tests/NodeRevisionsUiTest.php
@@ -119,17 +119,17 @@ public function testNodeRevisionDoubleEscapeFix() {

@@ -119,17 +119,17 @@ public function testNodeRevisionDoubleEscapeFix() {
     // Assert the old revision message.
     $date = format_date($nodes[0]->revision_timestamp->value, 'short');
     $url = new Url('entity.node.revision', ['node' => $nodes[0]->id(), 'node_revision' => $nodes[0]->getRevisionId()]);
-    $old_revision_message = t('!date by !username', [
-      '!date' => \Drupal::l($date, $url),
-      '!username' => $editor,
+    $old_revision_message = t('@date by @username', [
+      '@date' => \Drupal::l($date, $url),
+      '@username' => $editor,
     ]);
     $this->assertRaw($old_revision_message);

     // Assert the current revision message.
     $date = format_date($nodes[1]->revision_timestamp->value, 'short');
-    $current_revision_message = t('!date by !username', [
-      '!date' => $nodes[1]->link($date),
-      '!username' => $editor,
+    $current_revision_message = t('@date by @username', [
+      '@date' => $nodes[1]->link($date),
+      '@username' => $editor,
     ]);
     $current_revision_message .= '<p class="revision-log">' . $revision_log . '</p>';
     $this->assertRaw($current_revision_message);

This test also needs a followup. It's supposed to be a test for a double-escaping fix, but it does an assertRaw() on the result of t() rather than asserting the actual raw expected output. This means that if our changes to t() end up reintroducing double-escaping, this test will just test that it's double-escaped.

Proposed resolution

1. Remove t() and assert raw expected text.
2. Add an assertion to find double escaping.

Remaining tasks

User interface changes

Ideally, fewer completely unexpected double-escaping bugs.

API changes

None.