Add server_mount_fuzzer for static file serving (#2666)

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.

test/fuzzing/server_mount_fuzzer.dict

@@ -0,0 +1,24 @@
+"GET"
+"HEAD"
+"HTTP/1.1"
+
+"/mnt/"
+"/mnt/index.html"
+"/mnt/a.txt"
+"/mnt/data.bin"
+"/mnt/sub/"
+"/mnt/link-in.txt"
+"/mnt/link-out.txt"
+"/mnt/link-sibling.txt"
+".."
+"%2e%2e"
+"%2f"
+"//"
+
+"Range: bytes=0-"
+"Range: bytes=0-3,5-"
+"If-None-Match: "
+"W/"
+"If-Modified-Since: "
+"If-Range: "
+"Accept-Encoding: gzip"