Move `PropSourceEndpointTest` into new `XbConfigEntityHttpApiTest::testComponent()`

Overview

@tedbow was forced to introduce PropSourceEndpointTest in 📌 HTTP API: update /xb-component/{component_id} to list possible prop sources for current entity context Fixed , because no functional tests existed for that internal HTTP API response yet.

That was merged August 29, 2024.

ApiConfigControllers and its corresponding functional test coverage at XbConfigEntityHttpApiTest were both introduced in 📌 HTTP API to read+write PageTemplate and Pattern config entities Active by yours truly on October 22, 2024, in an attempt to curb the explosion in number of (routing) controllers.
Months later, that attempt has proven succesful 👍

However, the one outlier now is PropSourceEndpointTest.

Proposed resolution

The PropSourceEndpointTest test coverage ought to be moved into XbConfigEntityHttpApiTest, and match the existing naming scheme, so it should be in a test method named ::testComponent().

User interface changes

None.