Add tests for keyed and unkeyed int types

[hodovani]

No description provided.

[MaxDesiatov]

thanks @hodovani! great stuff in general, but I think UnkeyedIntTests has a lot of duplicated code that could be made generic similar to what we have in KeyedIntTests in this same PR

[MaxDesiatov]

could we create a new testIntArray generic function here and call that function with a metatype argument similar to what is available in KeyedIntTests?

[codecov]

Codecov Report

Merging #62 into master will increase coverage by 2.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   68.68%   70.76%   +2.08%     
==========================================
  Files          30       30              
  Lines        1440     1440              
==========================================
+ Hits          989     1019      +30     
+ Misses        451      421      -30

Impacted Files	Coverage Δ
Sources/XMLCoder/Auxiliaries/XMLElement.swift	95.12% <0%> (+0.81%)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	85.1% <0%> (+6.38%)	⬆️
...XMLCoder/Decoder/XMLUnkeyedDecodingContainer.swift	24.42% <0%> (+15.26%)	⬆️

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