WorkflowMessageTest - Re-enable skipped test

[totten]

Overview

This is a follow-up to #23811 (409b1e0). The prior commit changed the way it found examples - and caused it to skip some examples.

This re-enables the example and adds an assertion to ensure that it actually finds some examples.

Before

• Test may be skipped -- because data-provider may return empty.

After

• Test runs -- because data-provider returns real data.
• If data-provider somehow returns empty, then it would raise a different error.

[civibot]

(Standard links)

• If this is your first pull-request for CiviCRM, please browse CONTRIBUTING.md for information about the development and testing processes.
• If you are reviewing this pull-request, you may wish to consult the test sites and the Review Standards (long template, short template).