Most tests in SourceKit-LSP are integration tests that create a SourceKitLSPServer instance in-process, initialize it and send messages to it. The test support modules essentially define four ways of creating test projects.
Launches a SourceKitLSPServer in-process. Documents can be opened within it but these documents don't have any representation on the file system. TestSourceKitLSPClient has the lowest overhead and is the basis for all the other test projects. Because there are no files on disk, this type cannot test anything that requires cross-file functionality or exercise requests that require an index.
Creates a single .swift file on disk, indexes it and then opens it using a TestSourceKitLSPClient. This is the best choice for tests that require an index but don’t need to exercise any cross-file functionality.
Creates a SwiftPM project on disk that allows testing of cross-file and cross-module functionality. By default the SwiftPMTestProject does not build an index or build any Swift modules, which is often sufficient when testing cross-file functionality within a single module. When cross-module functionality or an index is needed, background indexing can be enabled using enableBackgroundIndexing: true, which waits for background indexing to finish before allowing any requests.
This is the most flexible test type that writes arbitrary files to disk. It provides less functionality out-of-the-box but is capable of eg. representing workspaces with multiple SwiftPM projects or projects that have compile_commands.json.