Change get_balances signature and refactor tests

[Levi0804]

Following the discussion in #385 (comment), this PR makes the necessary changes.

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.

Project coverage is 70.52%. Comparing base (d581535) to head (7e8da53).
Report is 22 commits behind head on master.

Files with missing lines	Patch %	Lines
src/wallet/api.rs	69.23%	4 Missing ⚠️
src/maker/rpc/server.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
+ Coverage   70.42%   70.52%   +0.10%     
==========================================
  Files          34       34              
  Lines        4247     4248       +1     
==========================================
+ Hits         2991     2996       +5     
+ Misses       1256     1252       -4     

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[mojoX911]

very clean changes, thanks for taking it up. Ack.

[Levi0804]

Happy to contribute! :D

[mojoX911]

Ack