Upgrade tests to be compatible with path_alias module in D8.8

From #3070671: Drupal 9 Deprecated Code Report:

@phenaproxima:

Hmm...it looks like the test will need to be updated for compatibility with the path_alias module in D8.8. I'll leave that to someone else who knows more than I do, and possibly a follow-up issue.

@wim-leers:

+++ b/tests/src/Functional/RequestTest.php
@@ -373,6 +373,11 @@ class RequestTest extends BrowserTestBase {
+    // @todo Update the expectations to include path alias stuff, and then
+    // delete these unset() calls.
+    unset($decoded_response['definitions']['path_alias--path_alias']);
+    unset($decoded_response['paths']['/path_alias/path_alias']);
+    unset($decoded_response['paths']['/path_alias/path_alias/{entity}']);

This is because in Drupal 8.8, path aliases became entities. Makes sense to not fix that here, to keep scope tight.