TestHostDocument doesn't support multiple positions

[Youssef1313]

roslyn/src/EditorFeatures/TestUtilities/Workspaces/TestWorkspace_XmlConsumption.cs

Lines 783 to 784 in 67dafdb

	TestFileMarkupParser.GetPositionAndSpans(markupCode,
	out var code, out int? cursorPosition, out ImmutableDictionary<string, ImmutableArray<TextSpan>> spans);

roslyn/src/EditorFeatures/TestUtilities/Workspaces/TestWorkspace_XmlConsumption.cs

Lines 797 to 798 in 67dafdb

	return new TestHostDocument(
	exportProvider, languageServiceProvider, code, fileName, fileName, cursorPosition, spans, codeKind, folders, isLinkFile, documentServiceProvider);

Instead of GetPositionAndSpans, I want to use GetPositionsAndSpans, but TestHostDocument doesn't accept multiple positions. This blocks adding a test for #54741.

@sharwell