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% 🎉

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

📣 This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

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%)	⬆️
lib/primitives/headers.js	77.55% <0.00%> (+4.08%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.