Reimplement main aspect of assertEqualXMLStructure

[theseer]

As discussed in #4091, there is a valid use case for having an assertion ensuring two DOM sub trees are structurally identical.

This PR re-implements the main aspect of the original, deprecated assertEqualXMLStructure: The comparison of two XML trees for structural identicality.

Two DOM trees are to be considered identical when, after canonicalization, the following holds true:

• The same child - parent relationship is maintained
• The same attributes are set on a node
• The attribute values are identical
• The textual content of nodes are identical

Looking at the options of the original, deprecated method, this appears to be stricter. The original method for instance ignored namespace defining attributes as well as textual content.

As I didn't see a use case for that, I did not reimplement that aspect. My implementation basically ignores any whitespace and, due to canonicalization, reorders attributes as needed so the order of attributes is not relevant.

The name assertDOMTreesEqualStructurally is subject to change, only pending a better naming suggestion. It should probably reflect the actual assertion better.

[codecov]

Codecov Report

Merging #4507 into master will decrease coverage by 0.09%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #4507      +/-   ##
============================================
- Coverage     83.69%   83.59%   -0.10%     
- Complexity     4663     4664       +1     
============================================
  Files           293      293              
  Lines         12030    12044      +14     
============================================
  Hits          10068    10068              
- Misses         1962     1976      +14     

Impacted Files	Coverage Δ	Complexity Δ
src/Framework/Assert.php	91.21% <0.00%> (-1.82%)	231.00 <1.00> (+1.00)	⬇️

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[sebastianbergmann]

This branch has conflicts that must be resolved.