Disable failing TCP server test

[zugz]

The test's intermittent failure may well be exposing a real bug in the
TCP relay and/or onion systems, but we can't find the bug, and keeping
the test is disrupting our CI.

---

This change is 

[zoff99]

@zugz

./auto_tests/tox_many_tcp_test.c:27:6: error: no previous extern declaration for
      non-static variable 'enable_broken_tests'
      [-Werror,-Wmissing-variable-declarations]
bool enable_broken_tests = false;

[sudden6]

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

[codecov]

Codecov Report

Merging #1311 into master will decrease coverage by 0.2%.
The diff coverage is 50%.

@@           Coverage Diff            @@
##           master   #1311     +/-   ##
========================================
- Coverage    84.4%   84.2%   -0.3%     
========================================
  Files          85      85             
  Lines       15442   15443      +1     
========================================
- Hits        13047   13008     -39     
- Misses       2395    2435     +40

Impacted Files	Coverage Δ
auto_tests/tox_many_tcp_test.c	56.1% <50%> (-43.9%)	⬇️
toxcore/ping_array.c	91.9% <0%> (-3.3%)	⬇️
toxcore/onion_client.c	95.6% <0%> (-0.8%)	⬇️
toxcore/TCP_connection.c	77.7% <0%> (-0.6%)	⬇️
toxcore/net_crypto.c	94.6% <0%> (-0.4%)	⬇️
toxav/toxav.c	68.7% <0%> (-0.2%)	⬇️
toxcore/DHT.c	77.7% <0%> (-0.1%)	⬇️
toxcore/Messenger.c	86.9% <0%> (-0.1%)	⬇️
auto_tests/toxav_many_test.c	97.2% <0%> (+0.6%)	⬆️
... and 4 more

---

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