test: added more coverage to tests

[7suyash7]

Added some more tests for:

1. coin-test.js
2. coins-test.js
3. consensus-test.js
4. gcs-test.js
5. headers-test.js
6. mnemonic-test.js
7. mtx-test.js
8. pow-test.js

[codecov-commenter]

Codecov Report

Base: 69.53% // Head: 69.54% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (994d064) compared to base (57f5956). Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1134      +/-   ##
==========================================
+ Coverage   69.53%   69.54%   +0.01%     
==========================================
  Files         158      158              
  Lines       26598    26598              
==========================================
+ Hits        18494    18497       +3     
+ Misses       8104     8101       -3     

Impacted Files	Coverage Δ
lib/hd/mnemonic.js	82.65% <0.00%> (+0.51%)	:arrow_up:
lib/primitives/headers.js	77.55% <0.00%> (+4.08%)	:arrow_up:

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