CI: Webserver core test might not work

The webserver core test might not work because it uses an already existing port.
This can probably be fixed by setting the port to 0 and then querying it afterwards.
Example of error:
`1/2 Test #1: unit-test-osmc-core ..............***Failed 0.47 sec`

Start by running the unit-test-osmc-core test and inspect the webserver core test that produces the reported port-conflict failure. Trace how its port is selected and queried; done means the test no longer depends on an already occupied port and passes reliably.

Written by the indexing model from the issue text.