* Added check to only find .css files in FileSystemTreeServiceBase.cs
* Marking GetFiles as virtual and overriding it in StyleSheetTreeService.cs to only find .css files
* Redone tests to fit new format
* Fix tests to use file extensions
* Adding file extensions to all other relevant tests
* Adding file filter to remaining trees
* Adding tests to ensure invalid filetypes wont show
* Encapulation and resolved minor warnings in tests.
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Co-authored-by: Andy Butland <abutland73@gmail.com>
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