tests: Bluetooth: df: connectionless cte tx migrate to new ZTEST API

Migrate tests in connectionless_cte_tx to new ZTEST API.

Remove main.c source file because is unncessary when automatic
unit tests registration is available. Also former use of
predicate to select unit tests for execution is removed
because the tests are split into two distinct tests suites
that has its't own setup and teardown phases.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>