[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.

[maxsharabayko]

[ RUN      ] Bonding.ConnectNonBlocking
Created group socket: 1428448644
Connecting two sockets
[A] Waiting for accept
Connect callback. Socket: 354706819, error: 0, token: 3
Returned from connecting two sockets 
/Users/travis/build/Haivision/srt/test/test_bonding.cpp:162: Failure
Expected equality of these values:
  ev[0].events
    Which is: 17
  SRT_EPOLL_IN
    Which is: 1
Connect callback. Socket: 354706818, error: 0, token: 4
[  FAILED  ] Bonding.ConnectNonBlocking (1188 ms)

[ethouris]

LOL actually didn't expect that the uwait might report IN and UPDATE in one. That case is also to be tolerated, I'll make a fix.

[codecov]

Codecov Report

Merging #1559 (5834866) into master (c065d88) will increase coverage by 2.75%.
The diff coverage is 90.51%.

@@            Coverage Diff             @@
##           master    #1559      +/-   ##
==========================================
+ Coverage   60.74%   63.50%   +2.75%     
==========================================
  Files          78       78              
  Lines       17444    17762     +318     
==========================================
+ Hits        10596    11279     +683     
+ Misses       6848     6483     -365     

Impacted Files	Coverage Δ
srtcore/group.h	74.48% <ø> (+17.24%)	⬆️
srtcore/group.cpp	57.09% <53.96%> (+20.67%)	⬆️
test/test_bonding.cpp	96.36% <96.23%> (-2.49%)	⬇️
srtcore/api.cpp	50.21% <100.00%> (+1.49%)	⬆️
srtcore/core.cpp	55.66% <100.00%> (+0.36%)	⬆️
srtcore/list.cpp	76.81% <0.00%> (-5.51%)	⬇️
srtcore/congctl.cpp	81.02% <0.00%> (-3.08%)	⬇️
srtcore/core.h	80.64% <0.00%> (-2.16%)	⬇️
srtcore/window.cpp	81.81% <0.00%> (-1.30%)	⬇️
... and 14 more

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