Added unit tests for array and dictionary properties

[regexident]

Array:

• empty array
• single element array
• multi element array

Dictionary:

• empty dictionary
• single element dictionary
• multi element dictionary

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(A checkmark means the test runs successfully with the current implementation)

[codecov]

Codecov Report

Merging #7 into master will increase coverage by 0.68%.
The diff coverage is 49.01%.

@@            Coverage Diff             @@
##           master       #7      +/-   ##
==========================================
+ Coverage   38.68%   39.36%   +0.68%     
==========================================
  Files          14       14              
  Lines        1440     1443       +3     
==========================================
+ Hits          557      568      +11     
+ Misses        883      875       -8

Impacted Files	Coverage Δ
Sources/XMLCoder/Encoder/XMLEncoder.swift	43.43% <0%> (ø)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	34.79% <33.33%> (+1.86%)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	27.93% <44.44%> (+1.7%)	⬆️
Sources/XMLCoder/Encoder/XMLEncodingStorage.swift	88.88% <75%> (-11.12%)	⬇️
Sources/XMLCoder/Decoder/XMLDecodingStorage.swift	81.81% <80%> (ø)	⬆️
Sources/XMLCoder/XMLStackParser.swift	93.8% <0%> (+0.95%)	⬆️

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