Tests: Bluetooth: Mesh: Refactor and cleanup of test_brg

This introduces a global variable controlling the number of remote nodes
used in a given test case. This allows removal of the `TEST_TYPE_...`
enums and further tests in the future that can use different remote node
counts.

This also cleans up the test source by extracting common tester
initialization to separate functions, avoiding repetition.

Signed-off-by: Ludvig Jordet <ludvig.jordet@nordicsemi.no>