selftests: forwarding: bridge_fdb_learning_limit: Add a new selftest

Add a suite covering the fdb_n_learned and fdb_max_learned bridge
features, touching all special cases in accounting at least once.

Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Johannes Nixdorf <jnixdorf-oss@avm.de>
Link: https://lore.kernel.org/r/20231016-fdb_limit-v5-5-32cddff87758@avm.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>