Rename all the classes that write files to disk to create a test project that we can open in sourcekit-lsp to end with `TestProject`. This is better than the old `Workspace` suffix because it avoids ambiguities with the `Workspace` type inside sourcekit-lsp.
- IndexedSingleSwiftFileWorkspace -> IndexedSingleSwiftFileTestProject
- MultiFileTestWorkspace -> MultiFileTestProject
- SwiftPMTestWorkspace -> SwiftPMTestProject
