The Blazor test infrastructure made a copy of the code in Razor and then
added some more features. This change backports the features needed for
the style of test we're using in Blazor.
I updated the MVC integration tests to use the new style, but I think
there's limited value in trying to rev all of the old tests.
One feature in particular that I removed from the old infrastructure was
the automatic inference of imports based on the file system. This
feature was wierd and doesn't parallel how these features work in
actuality. It's easy and more natural to test imports in new style
tests.
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