refactor(InMemorySinger) - test and test_signer methods application

[mkamonMdt]

Two factory methods for InMemorySigner were provided in #12503. The following commit applies the methods broadly, reducing verbose test code where possible.

[mkamonMdt]

Hi, @wacban
That is the followup commit on InMemorySigner::test/test_signer methods refactor :)

[wacban]

LGTM, thank you for looking into it!

Some CI is failing in the nightly run, please fix and I'll merge it then. Repro should be something like the following:
just nextest nightly-slow

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 70.04%. Comparing base (0194311) to head (a720786).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
test-utils/runtime-tester/src/scenario_builder.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12537      +/-   ##
==========================================
- Coverage   70.05%   70.04%   -0.01%     
==========================================
  Files         840      840              
  Lines      169738   169713      -25     
  Branches   169738   169713      -25     
==========================================
- Hits       118911   118882      -29     
- Misses      45690    45695       +5     
+ Partials     5137     5136       -1     

Flag	Coverage Δ
backward-compatibility	0.16% <0.00%> (ø)
db-migration	0.16% <0.00%> (ø)
genesis-check	1.29% <0.00%> (+<0.01%)	⬆️
linux	69.31% <81.25%> (+0.01%)	⬆️
linux-nightly	69.60% <81.25%> (-0.03%)	⬇️
pytests	1.59% <0.00%> (+<0.01%)	⬆️
sanity-checks	1.40% <0.00%> (+<0.01%)	⬆️
unittests	69.86% <81.25%> (-0.01%)	⬇️
upgradability	0.20% <0.00%> (ø)

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