tests: Unit test for wallet/direct_send.rs

[wthrajat]

Aims to fix #83, currently WIP.
Refactor the APIs for slight logic improvements and better error handling.
Also the previous tests are extended for various edge cases.

fn create_direct_send()

^ is also implemented for the unit test - after which the whole module is covered keeping our codecov in mind.

[codecov]

Codecov Report

Attention: Patch coverage is 51.08696% with 45 lines in your changes are missing coverage. Please review.

Project coverage is 73.05%. Comparing base (e794ff0) to head (a1d1e3e).
Report is 1 commits behind head on master.

❗ Current head a1d1e3e differs from pull request most recent head 808f62e. Consider uploading reports for the commit 808f62e to get more accurate results

Files	Patch %	Lines
src/wallet/direct_send.rs	51.64%	44 Missing ⚠️
src/wallet/api.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #99      +/-   ##
==========================================
- Coverage   78.64%   73.05%   -5.59%     
==========================================
  Files          25       28       +3     
  Lines        6045     6194     +149     
==========================================
- Hits         4754     4525     -229     
- Misses       1291     1669     +378     

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[rajarshimaitra]

Please rebase after #100 merge to pick up the clippies.

[rajarshimaitra]

Closing this as the internals of direct send api changed and a test to spend swap coins are added. In #112