Add test coverage

[s-a-tanjim]

• Add test on self_signing derivation
• Add test on basic derivation

[kentbull]

Nice work @s-a-tanjim !

[m00sey]

if you pull and run cargo fmt this will fix your build error.

[codecov-commenter]

Codecov Report

Merging #9 (e28ffe3) into main (2833777) will increase coverage by 0.71%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   71.77%   72.48%   +0.71%     
==========================================
  Files          13       13              
  Lines         418      418              
==========================================
+ Hits          300      303       +3     
+ Misses        118      115       -3     

Impacted Files	Coverage Δ
src/derivation/attached_signature_code.rs	84.44% <ø> (+2.22%)	:arrow_up:
src/derivation/basic.rs	97.29% <ø> (ø)
src/derivation/self_signing.rs	82.75% <ø> (+6.89%)	:arrow_up:

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