[TESTS] Added unit tests for bonding

[ethouris]

Added tests:

TEST(Bonding, ConnectBlind)

Connect to a nonexistent endpoint and interrupt it by closing a group.

TEST(Bonding, ConnectNonBlocking)

Test connecting with non-blocking mode and correctly reported epoll events.

TEST(Bonding, BackupPriorityBegin)

Setup two links with different weight and expect the one with higher priority to be activated.

TEST(Bonding, BackupPriorityTakeover)

Setup a new link while in transmission and expect the new link with higher priority takes over, even if there's no need to backup.

TEST(Bonding, BackupPrioritySelection)

Setup two links with high priority and send a packet to activate one. Then connect a link with low priority, kill the active link and send the packet - expected is that the out of two remaining link, the one with higher priority is activated.