Add a framework for LSP testing. (#4841)

Also adds tests for exit and initialize, just basic things.

toolchain/language_server/language_server.cpp

@@ -6,6 +6,7 @@
 
 #include "clang-tools-extra/clangd/LSPBinder.h"
 #include "clang-tools-extra/clangd/Transport.h"
+#include "clang-tools-extra/clangd/support/Logger.h"
 #include "common/raw_string_ostream.h"
 #include "toolchain/language_server/context.h"
 #include "toolchain/language_server/incoming_messages.h"
@@ -14,7 +15,12 @@
 namespace Carbon::LanguageServer {
 
 auto Run(FILE* input_stream, llvm::raw_ostream& output_stream,
-         llvm::raw_ostream& /*error_stream*/) -> ErrorOr<Success> {
+         llvm::raw_ostream& error_stream) -> ErrorOr<Success> {
+  // TODO: Consider implementing a custom logger that splits vlog to
+  // vlog_stream when provided. For now, this disables verbose logging.
+  clang::clangd::StreamLogger logger(error_stream, clang::clangd::Logger::Info);
+  clang::clangd::LoggingSession logging_session(logger);
+
   // Set up the connection.
   std::unique_ptr<clang::clangd::Transport> transport(
       clang::clangd::newJSONTransport(input_stream, output_stream,