Extra Decred signer tests and fixes

[optout21]

Description

Extra tests for Decred signer for increased coverage.

Testing instructions

Unit tests.

Types of changes

• Extra tests
• Minor fixes

Checklist

• Prefix PR title with [WIP] if necessary.
• Add tests to cover changes as needed.
• Update documentation as needed.
• If there is a related Issue, mention it in the description (e.g. 'Fixes Adopt Zilliqa to bech32 address format #444 ).

[hewigovens]

forget to check in new added files?

[codecov]

Codecov Report

Merging #949 into master will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
+ Coverage   93.49%   93.77%   +0.27%     
==========================================
  Files         425      425              
  Lines       12230    12213      -17     
==========================================
+ Hits        11435    11453      +18     
+ Misses        795      760      -35     

Impacted Files	Coverage Δ
src/Bitcoin/TransactionSigner.h	100.00% <ø> (ø)
src/Decred/Signer.h	100.00% <ø> (ø)
src/Decred/TransactionBuilder.h	95.00% <ø> (-0.24%)	⬇️
src/Bitcoin/TransactionSigner.cpp	87.34% <100.00%> (+6.25%)	⬆️
src/Decred/Signer.cpp	70.96% <100.00%> (+14.62%)	⬆️
src/interface/TWBitcoinScript.cpp	96.42% <0.00%> (-3.58%)	⬇️
src/Bitcoin/Transaction.cpp	97.63% <0.00%> (-2.37%)	⬇️
src/Result.h	81.48% <0.00%> (+18.51%)	⬆️

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