Semantic snippets tests

[DoctorKrolic]

Fixes: #73909

Remade tests for semantic snippets from scratch:

• Now semantic snippets are tested independently from completions. There were a few completion-related tests, moved them to SemanticSnippetCompletionProviderTests. If we need to test that integration point more in the future, we can add new tests to this class
• As a result of decoupling new tests moved from EditorFeatures to Features layer
• We can now also verify placeholders. They are specified in the markup as annotated spans {|0:placeholder|}, {|1:placeholder2|} etc.
• Instead of workspace xml, which is hard to read, we now pass editorconfig and reference assemblies separately (I cannot think of a scenario where semantic snippets can need more advanced xml features, such as multiple documents and so on)
• Some tests got simplified (e.g. for/forr test instead of testing Length and Count property separately now use combinatorial data), some got renamed for consistency accross all semantic snippet tests
• Reused some common test data instead of duplicating [InlineData] blocks
• Due to Semantic snippets test flaw #73909 there was a bug, which sneaked in unnoticed. Was a trivial fix, so included it into this PR

[DoctorKrolic]

@akhera99 PTAL

[akhera99]

Looks really good, thank you for your continued contributions in this space!

[DoctorKrolic]

Someone restart failed pipelines pls