Run all tests (and compilation) in parallel with autotools.

[iphydf]

With distcheck, it's all or nothing, so we build with -j50 and run tests
with -j50.

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This change is 

[codecov]

Codecov Report

Merging #1122 into master will decrease coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1122     +/-   ##
========================================
- Coverage    82.5%   82.4%   -0.2%     
========================================
  Files          80      80             
  Lines       14339   14339             
========================================
- Hits        11838   11821     -17     
- Misses       2501    2518     +17

Impacted Files	Coverage Δ
auto_tests/toxav_many_test.c	92.5% <0%> (-4.1%)	⬇️
toxcore/network.c	83.4% <0%> (-1.5%)	⬇️
toxcore/LAN_discovery.c	83.9% <0%> (-1%)	⬇️
toxcore/tox.c	65.7% <0%> (-1%)	⬇️
toxcore/friend_connection.c	93.6% <0%> (-0.9%)	⬇️
toxcore/net_crypto.c	93.1% <0%> (-0.5%)	⬇️
toxcore/Messenger.c	85.8% <0%> (-0.4%)	⬇️
toxav/toxav.c	68.8% <0%> (ø)	⬆️
toxav/msi.c	65.9% <0%> (+0.8%)	⬆️
toxcore/group.c	73.1% <0%> (+1.4%)	⬆️

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[robinlinden]

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