Fix test class name for mono_time_test.

[iphydf]

Was: Util, should be: MonoTime.

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This change is 

[codecov]

Codecov Report

Merging #1130 into master will decrease coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1130     +/-   ##
========================================
- Coverage    82.5%   82.4%   -0.1%     
========================================
  Files          81      81             
  Lines       14415   14415             
========================================
- Hits        11894   11891      -3     
- Misses       2521    2524      +3

Impacted Files	Coverage Δ
toxcore/mono_time_test.cc	100% <100%> (ø)	⬆️
toxcore/LAN_discovery.c	82% <0%> (-1.9%)	⬇️
toxcore/group.c	71.7% <0%> (-1.5%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-0.9%)	⬇️
toxav/msi.c	65.3% <0%> (-0.3%)	⬇️
toxcore/Messenger.c	85.7% <0%> (-0.2%)	⬇️
toxcore/net_crypto.c	93.6% <0%> (ø)	⬆️
toxav/toxav.c	68.9% <0%> (+0.3%)	⬆️
toxcore/onion_client.c	95% <0%> (+0.7%)	⬆️
auto_tests/toxav_basic_test.c	83.5% <0%> (+1.9%)	⬆️
... and 1 more

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