Add cmake option for building additional tests

[zugz]

Closes #1262

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This change is 

[sudden6]

Reviewed 3 of 3 files at r1.
Reviewable status: 0 of 1 approvals obtained

[nurupo]

Did you test if it fixes the build failure with the STRICT_ABI flag?

It would be nice to print a message for every option we are overriding, e.g. if STRICT_ABI=ON and AUTOTEST=ON do message(WARNING "AUTOTEST option is not compatible with STRICT_ABI option. Disabling AUTOTEST"). Someone might get confused as to why autotests didn't build when they asked for them to be built.

[codecov]

Codecov Report

Merging #1278 into master will decrease coverage by 0.4%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1278     +/-   ##
========================================
- Coverage    83.7%   83.3%   -0.5%     
========================================
  Files          82      82             
  Lines       14827   14827             
========================================
- Hits        12416   12351     -65     
- Misses       2411    2476     +65

Impacted Files	Coverage Δ
toxcore/TCP_client.c	64.6% <0%> (-2.9%)	⬇️
toxav/msi.c	63.9% <0%> (-2.8%)	⬇️
toxcore/TCP_server.c	80.3% <0%> (-2.6%)	⬇️
toxav/toxav.c	67.7% <0%> (-1.5%)	⬇️
toxcore/friend_connection.c	94.9% <0%> (-1.2%)	⬇️
toxcore/DHT.c	76.8% <0%> (-1%)	⬇️
toxcore/network.c	82.1% <0%> (-0.5%)	⬇️
toxcore/net_crypto.c	95.1% <0%> (-0.3%)	⬇️
toxcore/Messenger.c	86.7% <0%> (-0.3%)	⬇️
toxcore/onion_client.c	96% <0%> (-0.2%)	⬇️

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[zugz]

* Sunday, 2018-12-09 at 16:08 -0800 - nurupo <notifications@github.com>:

Did you test if it fixes the build failure with the `STRICT_ABI` flag?

Yes, it works fine for me.

It would be nice to print a message for every option we are overriding,

Good idea. I've implemented that now (though if someone knows cmake and can avoid the code duplication involved, that would be nice).

[hugbubby]

@zugs You need to sign your commits with a GPG key before they can be merged. I know it's possible for me to sign them because iphydf did that for me once, but I don't know how.