Assert and Application fluent testing support

[tznind]

[...], I think it would be great if there was a GuiTestContext member called Assert that somehow neatly integrated with xUnit so:

.SendKey (MenuBarv2.DefaultKey)
-.Then (() => Assert.True (Application.Popover?.GetActivePopover () is PopoverMenu))
-.Then (() => Assert.True (menuBar?.IsOpen()))
-.Then (() => Assert.Equal ("_New file", Application.Navigation?.GetFocused ()!.Title))
+.Assert.True (Application.Popover?.GetActivePopover () is PopoverMenu)
+.Assert.True (menuBar?.IsOpen())
+.Assert.Equal ("_New file", Application.Navigation?.GetFocused ()!.Title)
.ScreenShot ($"After {MenuBarv2.DefaultKey}", _out)

(Or roughly equivalent that is much cleaner).

Originally posted by @tig in #4015 (comment)