tests: Bluetooth: Mesh: add LCD metadata test. Refactor existing tests.

* Test that the Large Composition Data server can send a max access
payload metadata status message.
* Test that the Large Composition Data client can request large chunks
of metadata from the server, and correctly merge samples.
* Refactoring of existing LCD tests.

Signed-off-by: Stine Akredalen <stine.akredalen@nordicsemi.no>