selftests: net: bridge: factor out and rename sg state functions

Factor out S,G entry state checking functions for existence, forwarding,
blocking and timer to lib.sh so they can be later used by MLDv2 tests.
Add brmcast_ suffix to their name to make the relation to the bridge
explicit.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>