Include CTest in CMakeLists.txt to get valgrind support.

[iphydf]

We can now run ctest --output-on-failure -D ExperimentalMemCheck -j 50
to run valgrind on all tests.

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This change is 

[robinlinden]

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

[codecov]

Codecov Report

Merging #1229 into master will decrease coverage by <.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1229     +/-   ##
========================================
- Coverage    83.1%     83%   -0.1%     
========================================
  Files          82      82             
  Lines       14692   14647     -45     
========================================
- Hits        12210   12168     -42     
+ Misses       2482    2479      -3

Impacted Files	Coverage Δ
toxcore/TCP_server.c	75.9% <0%> (-4.4%)	⬇️
toxcore/LAN_discovery.c	84.9% <0%> (-1%)	⬇️
toxcore/Messenger.c	86.5% <0%> (-0.5%)	⬇️
toxcore/DHT.c	76.7% <0%> (-0.1%)	⬇️
toxcore/onion_client.c	95.3% <0%> (-0.1%)	⬇️
auto_tests/save_load_test.c	98.6% <0%> (+0.5%)	⬆️
auto_tests/toxav_many_test.c	97.2% <0%> (+0.6%)	⬆️
toxav/toxav.c	69.2% <0%> (+0.9%)	⬆️
toxcore/friend_connection.c	94.5% <0%> (+1.1%)	⬆️
toxav/msi.c	67% <0%> (+1.9%)	⬆️
... and 1 more

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