[tests] Minor fix for Bonding.CloseGroupAndSocket

[maxsharabayko]

In the Bonding.CloseGroupAndSocket unit test (If a listener is not available) the following IPE situation can happen when sockets already removed from the epoll are being removed again.
Added ASSERT_GT(epoll_res, 0) to fail the test if epoll returns an error.

[ RUN      ] Bonding.CloseGroupAndSocket
22: Created group socket: 1405307167
22: Connecting two sockets 
22: Socket created: 331565342
22: Listen: wait for acceptability
22: Socket created: 331565340
22: Returned from connecting two sockets 
22: Listen: reported 1 acceptable and 0 errors
22: Listener will read packets...
22: Epoll result: 2
22: Epoll rlen: 0, wlen: 2
22: Epoll write[0]: 331565342 (removed from epoll)
22: Epoll write[1]: 1405307167 (removed from epoll)
22: Connect callback. Socket: 331565340, error: 1001, token: 2
22: 13:40:54.880000/T6716*E:SRT.ea: EID:3 no sockets to check, this would deadlock
22: Epoll result: -1
22: Epoll rlen: 0, wlen: 2
22: Epoll write[0]: 331565342 (removed from epoll)
22: 13:40:54.880000/T6716*E:SRT.ei: epoll/update: IPE: update struck E3 which is NOT SUBSCRIBED to @331565342
22: Epoll write[1]: 1405307167 (removed from epoll)
22: 13:40:54.880000/T6716*E:SRT.ei: epoll/update: IPE: update struck E3 which is NOT SUBSCRIBED to @1405307167