Fuzz Server::handle_file_request through a mount point backed by a temporary directory that holds symlinks escaping the mount root, one of them into a sibling directory sharing its name prefix. Abort if a token stored outside the root appears in a response. The seeds are based on those of #2661.
Move FuzzedStream into fuzzed_stream.h so that both server fuzzers share it, and add FuzzableServer::MarkAsRunning(), without which a server that is not listening writes no file.
