Avoid use of IPv6 in tests if not supported (e.g. on Travis).

[iphydf]

This change is 

[codecov]

Codecov Report

Merging #1224 into master will decrease coverage by 0.5%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1224     +/-   ##
========================================
- Coverage    83.2%   82.6%   -0.6%     
========================================
  Files          82      81      -1     
  Lines       14647   14564     -83     
========================================
- Hits        12195   12040    -155     
- Misses       2452    2524     +72

Impacted Files	Coverage Δ
toxcore/crypto_core_test.cc	90.3% <0%> (-9.7%)	⬇️
toxav/msi.c	64.5% <0%> (-4%)	⬇️
toxcore/network.c	79.5% <0%> (-3.9%)	⬇️
toxav/toxav.c	67.1% <0%> (-2.9%)	⬇️
auto_tests/toxav_basic_test.c	81.6% <0%> (-2%)	⬇️
toxcore/friend_connection.c	94.5% <0%> (-1.5%)	⬇️
toxcore/LAN_discovery.c	84.9% <0%> (-1%)	⬇️
toxcore/net_crypto.c	94.7% <0%> (-0.4%)	⬇️
toxcore/Messenger.c	86.5% <0%> (-0.3%)	⬇️
toxcore/onion_client.c	95.9% <0%> (-0.2%)	⬇️

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