Remove unnecessary do … catch blocks in unit tests & add tests for missing values

[regexident]

No description provided.

[codecov]

Codecov Report

Merging #40 into master will increase coverage by 0.43%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
+ Coverage   51.32%   51.76%   +0.43%     
==========================================
  Files          29       29              
  Lines        1391     1391              
==========================================
+ Hits          714      720       +6     
+ Misses        677      671       -6

Impacted Files	Coverage Δ
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	42.85% <0%> (+4.08%)	⬆️

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[MaxDesiatov]

awesome, I didn't know that test functions can throw 👍