Fix discovery tests to accommodate changes in discovery registration

[Nadahar]

After the changes to Core in openhab/openhab-core#5032, some add-on tests failed because the timing of the registration process changed.

Some were taken care of in #19230, some will be fixed by #19471, some are yet to be fixed and were disabled and some seem to have slipped by unnoticed.

This addresses those that slipped by.

[Nadahar]

The changes to MQTT HomeAssistant don't seem to be directly related to openhab/openhab-core#5032, and as far as I can understand, the test must have been failing also in the past, if assertions were enabled.

I replaced assert with assertTrue() for the following reason: The tests actually have a system for waiting until the discovery results are in, using CountDownLatch, which should make it immune to the recent timing changes. But, for some reason, the waiting was done using assert, which is disabled by default and must be explicitly enabled when the JVM is launched. I don't know if this was intentional or not, but I see it as much more desirable that this waiting mechanism is enabled the whole time.

@jlaur FYI

[Nadahar]

@lsiepel I think this one should be good too.

[jlaur]

Thanks, LGTM.