`InvalidBlockSigByzantine` test: add `assertSameSignatures`

[hewison-chris]

This test checks the expected validator signatures are in the header of each valid node. The test was sometimes failing because the nodes had not all completed updating signatures from each other.

[codecov[bot]]

Codecov Report

Merging #3202 (6809da5) into v0.x.x (6e3a0e7) will decrease coverage by 7.29%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           v0.x.x    #3202      +/-   ##
==========================================
- Coverage   87.32%   80.03%   -7.30%     
==========================================
  Files         163      210      +47     
  Lines       16760    19064    +2304     
==========================================
+ Hits        14636    15257     +621     
- Misses       2124     3807    +1683     

Flag	Coverage Δ
integration	26.61% <ø> (?)
unittests	87.28% <100.00%> (-0.05%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
source/agora/test/InvalidBlockSigByzantine.d	96.29% <100.00%> (+0.14%)	:arrow_up:
source/agora/test/NetworkDiscovery.d	84.09% <0.00%> (-4.55%)	:arrow_down:
source/agora/consensus/state/ValidatorSet.d	84.83% <0.00%> (-1.13%)	:arrow_down:
source/agora/consensus/protocol/Nominator.d	91.87% <0.00%> (-0.38%)	:arrow_down:
source/agora/script/Engine.d	97.59% <0.00%> (-0.16%)	:arrow_down:
...ubmodules/ocean/src/ocean/util/ReusableException.d	0.00% <0.00%> (ø)
...gora/submodules/ocean/src/ocean/util/log/ILogger.d	0.00% <0.00%> (ø)
...ubmodules/ocean/src/ocean/text/convert/Formatter.d	56.28% <0.00%> (ø)
...ora/submodules/ocean/src/ocean/util/log/Appender.d	42.50% <0.00%> (ø)
...ra/submodules/ocean/src/ocean/text/convert/Float.d	0.00% <0.00%> (ø)
... and 53 more

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