Add XDocumentComparer and XmlComparer

[nowsprinting]

Add two NUnit test comparer implementations.

XDocumentComparer

XDocumentComparer is a NUnit test comparer class that loosely compares XDocument.

It only compares the attributes and values of each element in the document unordered.
XML declarations and comments are ignored.

XmlComparer

XmlComparer is a NUnit test comparer class that loosely compares string as XML documents.

It only compares the attributes and values of each element in the document unordered.
XML declarations and comments are ignored, and white spaces, tabs, and newlines before and after the value are ignored.

[github-actions]

Code Metrics Report

	master (ed7ebcf)	#105 (9852f26)	+/-
Coverage	72.3%	75.8%	+3.5%
Code to Test Ratio	1:0.9	1:0.8	-0.1
Test Execution Time	3m5s	3m47s	+42s

Details

  |                     | master (ed7ebcf) | #105 (9852f26) |  +/-  |
  |---------------------|------------------|----------------|-------|
+ | Coverage            |            72.3% |          75.8% | +3.5% |
  |   Files             |               27 |             29 |    +2 |
  |   Lines             |              668 |            806 |  +138 |
+ |   Covered           |              483 |            611 |  +128 |
- | Code to Test Ratio  |            1:0.9 |          1:0.8 |  -0.1 |
  |   Code              |             1291 |           1478 |  +187 |
+ |   Test              |             1207 |           1321 |  +114 |
- | Test Execution Time |             3m5s |          3m47s |  +42s |

Code coverage of files in pull request scope (0.0% → 92.7%)

Files	Coverage	+/-
Runtime/Comparers/XDocumentComparer.cs	92.0%	+92.0%
Runtime/Comparers/XmlComparer.cs	100.0%	+100.0%

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Reported by octocov