Add AUTOTEST option to CMakeLists.txt

[chron0]

This change is 

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[iphydf]

misc_tools is needed for other things than auto tests. Better add the condition to the existing if inside function(auto_test target).

[iphydf]

Why not add the condition inside the existing if?

[codecov]

Codecov Report

Merging #1223 into master will decrease coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1223     +/-   ##
========================================
- Coverage    82.9%   82.9%   -0.1%     
========================================
  Files          82      82             
  Lines       14647   14647             
========================================
- Hits        12152   12147      -5     
- Misses       2495    2500      +5

Impacted Files	Coverage Δ
toxav/toxav.c	67.6% <0%> (-1.7%)	⬇️
toxav/msi.c	65.6% <0%> (-1.2%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-0.9%)	⬇️
toxcore/Messenger.c	86.5% <0%> (-0.2%)	⬇️
toxcore/DHT.c	76.6% <0%> (-0.1%)	⬇️
toxcore/group.c	76.4% <0%> (+0.3%)	⬆️
toxcore/net_crypto.c	94.6% <0%> (+1%)	⬆️

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[iphydf]

Can you update README.md with the new option?

[nurupo]

Also enable autotests in https://github.com/TokTok/c-toxcore/blob/master/.travis/cmake-freebsd-stage2#L53