Make test output deterministic

[jkczyz]

Tests use sources of randomness to produce seeds, preimages, secrets,
and ephemeral data. However, this makes comparing logs between different
test runs difficult. Remove uses of random number generators and the
current time in favor of fixed values in order to make the test output
deterministic.

[codecov]

Codecov Report

Merging #641 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #641      +/-   ##
==========================================
+ Coverage   91.28%   91.30%   +0.02%     
==========================================
  Files          35       35              
  Lines       21138    21104      -34     
==========================================
- Hits        19296    19270      -26     
+ Misses       1842     1834       -8     

Impacted Files	Coverage Δ
lightning/src/lib.rs	100.00% <ø> (ø)
lightning/src/ln/channel.rs	86.78% <100.00%> (-0.03%)	⬇️
lightning/src/ln/channelmonitor.rs	95.70% <100.00%> (-0.02%)	⬇️
lightning/src/ln/functional_test_utils.rs	94.66% <100.00%> (-0.02%)	⬇️
lightning/src/ln/functional_tests.rs	97.29% <100.00%> (+0.17%)	⬆️
lightning/src/ln/peer_handler.rs	54.74% <100.00%> (-0.26%)	⬇️
lightning/src/util/test_utils.rs	85.20% <100.00%> (ø)
lightning/src/util/ser_macros.rs	96.59% <0.00%> (-0.69%)	⬇️
lightning/src/util/ser.rs	87.45% <0.00%> (-0.33%)	⬇️
... and 1 more

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[arik-so]

Looks great to me!