Convert Sample XML code to XCTest

[MaxDesiatov]

Currently most of the tests can be started with custom testing code in Sample XML directory. This prevents easy testing with SwiftPM and generation of code coverage. Tests should be migrated to XCTest to get true test coverage numbers.

Sample XML contents:

• Breakfast
• Books
• Plants
• RJI
• CDs
• Notes

[codecov-io]

Codecov Report

Merging #1 into master will increase coverage by 8.58%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   25.99%   34.57%   +8.58%     
==========================================
  Files          12       12              
  Lines        1585     1585              
==========================================
+ Hits          412      548     +136     
+ Misses       1173     1037     -136

Impacted Files	Coverage Δ
Sources/XMLCoder/Encoder/XMLEncoder.swift	29.88% <0%> (+6.95%)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	32.93% <0%> (+10.31%)	⬆️
Sources/XMLCoder/Encoder/XMLEncodingStorage.swift	100% <0%> (+12.5%)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	26.22% <0%> (+14.34%)	⬆️
Sources/XMLCoder/XMLStackParser.swift	92.85% <0%> (+17.14%)	⬆️

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