Use test clock in run_auto_test tests and dht test

[zugz]

This lets us test behaviours which involve timeouts without actually waiting
for the timeout to occur in realtime.

Based on #1038.

(This PR converts a few of the tests to use this facility; handling the
remainder is left to future work.)

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This change is 

[iphydf]

Looks good. Can you make the first commit ("make mono_time an argument") a separate PR?

[iphydf]

Can you also make the "rename monotime to mono_time" a separate PR?

[iphydf]

Reviewed 11 of 11 files at r1.
Reviewable status: 0 of 1 approvals obtained

[codecov]

Codecov Report

Merging #1095 into master will increase coverage by 0.3%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #1095     +/-   ##
========================================
+ Coverage    82.6%     83%   +0.3%     
========================================
  Files          82      82             
  Lines       14478   14473      -5     
========================================
+ Hits        11970   12014     +44     
+ Misses       2508    2459     -49

Impacted Files	Coverage Δ
auto_tests/overflow_sendq_test.c	100% <ø> (ø)	⬆️
auto_tests/friend_connection_test.c	100% <ø> (ø)	⬆️
auto_tests/run_auto_test.h	100% <100%> (ø)	⬆️
auto_tests/reconnect_test.c	100% <100%> (ø)	⬆️
auto_tests/overflow_recvq_test.c	100% <100%> (ø)	⬆️
auto_tests/send_message_test.c	94.1% <100%> (-0.7%)	⬇️
auto_tests/conference_peer_nick_test.c	100% <100%> (ø)	⬆️
auto_tests/dht_test.c	47.8% <100%> (+0.7%)	⬆️
auto_tests/conference_double_invite_test.c	100% <100%> (ø)	⬆️
auto_tests/lossy_packet_test.c	100% <100%> (ø)	⬆️
... and 13 more

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