Make the tsan build fail instead of swallowing its errors.

[iphydf]

We'll make it non-required, but we want to know about these failures so
we are incentivised to fix them.

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This change is 

[codecov]

Codecov Report

Merging #1133 into master will decrease coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1133     +/-   ##
========================================
- Coverage    82.6%   82.5%   -0.1%     
========================================
  Files          81      81             
  Lines       14437   14414     -23     
========================================
- Hits        11929   11904     -25     
- Misses       2508    2510      +2

Impacted Files	Coverage Δ
toxav/msi.c	64.2% <0%> (-2.6%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-0.9%)	⬇️
toxcore/DHT.c	76.6% <0%> (-0.9%)	⬇️
toxcore/network.c	83.4% <0%> (-0.3%)	⬇️
toxav/toxav.c	68.5% <0%> (-0.2%)	⬇️
toxcore/Messenger.c	85.7% <0%> (-0.1%)	⬇️
toxcore/mono_time_test.cc	100% <0%> (ø)	⬆️
toxcore/net_crypto.c	93.1% <0%> (ø)	⬆️
toxcore/LAN_discovery.c	84.9% <0%> (+2.8%)	⬆️
toxcore/mono_time.c	100% <0%> (+7.1%)	⬆️
... and 1 more

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