Implement support for <assert-xml file=".."/> for XSLT tests

[bojidar-bg]

This ends up fixing 54 tests across the board; out of a total of 537 tests that use <assert-xml file=".."/>.

Not convinced that's quite the place or way to implement the feature; just getting my feet wet for now 😅
I experimented with making a new AssertXmlFile struct which has a reimplementation of AssertXml::assert_value, but that causes more code duplication overall. I also played around with making assert_xml_query read the file directly, but then I cannot return a TestOutcome::EnvironmentError as easily. I also considered making AssertXml::MatchFile(...) hold a relative path and not an absolute path to the needed file, however getting the path to the testcases from assert_value is hard, given its current signature.

[faassen]

I wouldn't know where the right way is off the top of my head, but since it runs more tests successfully I'm taking it. It's not like the way we run the tests is the most important part of the codebase, as long as we can run them correctly.