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I generally try to ignore upstream bits. I've accidentally included some items from the `deps/` directory. I expect someone will give me a list of items to drop, I'm happy to drop whole files/directories, or to split the PR into multiple items (E.g. comments/locals/public).
Closes#4294
## Summary of the Pull Request
Fix the `TabTests`, and enable testing of types with XAML content. The `TabTests` were written many, many moons ago. they were intended to be our tests of XAML-like content within the Terminal app, so we could have unittests of Tabs, Panes, etc. Between their initial authoring and the day they were checked in, we had a bunch of build changes come in and break them irreperably.
We've gotten them fixed now with _one weird trick_ <sup>doctors hate me</sup>. As long as there isn't an `App.xbf` in the test's output directory, then the tests will deploy just fine.
We also needed a bit of magic, cribbed straight from TAEF, to enable running test code synchronously on the UI thread. Hence, `CppwinrtTailored.h`.
## References
## PR Checklist
* [x] Closes#2472
* [x] I work here
* [x] Tests added/passed - you better believe it
* [n/a] Requires documentation to be updated
## Validation Steps Performed
