test: unit tests for wallet/swapcoin.rs

[wthrajat]

Aims to fix #81.
Added unit tests for various methods like:

fn sign_transaction_input()
fn sign_hashlocked_transaction_input()
fn create_hashlock_spend_without_preimage()
fn apply_privkey_watchonly_swapcoin()
fn apply_privkey_incoming_swapcoin()
fn apply_privkey_outgoing_swapcoin()

Keeping our codecov in mind, various fail/error cases have also been covered to get the coverage higher. Also some tests are implemented for all three structs like IncomingSwapcoin, OutgoingSwapcoin, WatchonlySwapcoin to get more codecov coverage.

PS: After merging, we will go from 65% to 85% in this swapcoin.rs. Also these tests indirectly increased the coverage in several other files as well. We'll analyze the overall codecov once this is merged.

[codecov]

Codecov Report

Attention: Patch coverage is 90.68627% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 73.58%. Comparing base (a08292d) to head (451353d).

Files	Patch %	Lines
src/wallet/swapcoin.rs	90.68%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
+ Coverage   71.56%   73.58%   +2.02%     
==========================================
  Files          28       28              
  Lines        5911     6115     +204     
==========================================
+ Hits         4230     4500     +270     
+ Misses       1681     1615      -66     

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[rajarshimaitra]

Concept ACK. Please rebase after merge of #100 to pickup the clippy fixes.

[wthrajat]

Rebased after the merge of #100!

[rajarshimaitra]

ACK 451353d