selftests: forwarding: Rename bridge_mdb test

The test is only concerned with host MDB entries and not with MDB
entries as a whole. Rename the test to reflect that.

Subsequent patches will add a more general test that will contain the
test cases for host MDB entries and remove the current test.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>