bgpd: Improve test suite for peer-group overrides

bgpd: Improve test suite for peer-group overrides

This commit introduces the current test suite for BGP peer-group
overrides by adding support for custom check handlers (which can check
internal data structures more thoroughly) and by fixing several small
mistakes and issues that slipped through. Also some parts of the code
have been cleaned up to avoid duplicate and/or hard-to-read code.

Additionally a first experimental check for a BGP peer attribute with
values (advertisement-interval <value>) has been added to the test
suite. As this test suite is currently not passing, it has not been
added to the python test caller.

Signed-off-by: Pascal Mathis <mail@pascalmathis.com>