Call abort instead of exit on test failure.

[iphydf]

This raises a signal, so we can more easily catch it with gdb.

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This change is 

[codecov]

Codecov Report

Merging #1129 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1129   +/-   ##
======================================
  Coverage    82.4%   82.4%           
======================================
  Files          80      80           
  Lines       14345   14345           
======================================
  Hits        11823   11823           
  Misses       2522    2522

Impacted Files	Coverage Δ
toxcore/net_crypto.c	93.1% <0%> (-0.6%)	⬇️
toxcore/Messenger.c	85.8% <0%> (ø)	⬆️
toxav/msi.c	64.5% <0%> (+0.2%)	⬆️
toxav/toxav.c	68.1% <0%> (+0.6%)	⬆️

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