[System.XML] Add unit test for #18118
[System.XML] Fix handling of global variables in XSLs (#18118)
[System.XML] Add unit test for XSL note-set bug (#18114)
[System.XML] Fix XSL node-set bug (#18114)

XSL node-set function now behaves more like MS version.
[System.XML] Add unit test for XSL value-of bug (#18113)
[System.XML] Fix handling of space in XSL value-of elements (#18113)
Merge pull request #961 from ermshiperete/bug-xamarin-18118

[System.XML] XSL bugfixes for #18118 #18114 #18113