test: add fileutil to test

test

@@ -15,7 +15,7 @@ COVER=${COVER:-"-cover"}
 source ./build
 
 # Hack: gofmt ./ will recursively check the .git directory. So use *.go for gofmt.
-TESTABLE_AND_FORMATTABLE="client discovery error etcdctl/command etcdmain etcdserver etcdserver/etcdhttp etcdserver/etcdhttp/httptypes etcdserver/etcdserverpb integration migrate pkg/flags pkg/ioutils pkg/types pkg/transport pkg/wait proxy raft rafthttp snap store wal"
+TESTABLE_AND_FORMATTABLE="client discovery error etcdctl/command etcdmain etcdserver etcdserver/etcdhttp etcdserver/etcdhttp/httptypes etcdserver/etcdserverpb integration migrate pkg/fileutil pkg/flags pkg/ioutils pkg/types pkg/transport pkg/wait proxy raft rafthttp snap store wal"
 FORMATTABLE="$TESTABLE_AND_FORMATTABLE *.go etcdctl/"
 
 # user has not provided PKG override