Revert "test: Ignore "invalid non-UTF8 @DaTa passed" message"

That bug is fixed for good, so we should not run into this any more.
This also broke tests randomly due to pages receiving invalid messages.

This reverts commit ab17855.

test/common/testlib.py

@@ -1856,9 +1856,6 @@ def add_machine(
 
  # timedatex.service shuts down after timeout, runs into race condition with property watching
  ".*org.freedesktop.timedate1: couldn't get all properties.*Error:org.freedesktop.DBus.Error.NoReply.*",
-
- # https://github.com/cockpit-project/cockpit/issues/19235
- "invalid non-UTF8 @data passed as text to web_socket_connection_send.*",
  ]
 
  default_allowed_messages += os.environ.get("TEST_ALLOW_JOURNAL_MESSAGES", "").split(",")