test: activate test on storage/backend

test

@@ -16,7 +16,7 @@ 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/auth etcdserver/etcdhttp etcdserver/etcdhttp/httptypes pkg/fileutil pkg/flags pkg/idutil pkg/ioutil pkg/netutil pkg/osutil pkg/pbutil pkg/types pkg/transport pkg/wait proxy raft snap storage store version wal"
+TESTABLE_AND_FORMATTABLE="client discovery error etcdctl/command etcdmain etcdserver etcdserver/auth etcdserver/etcdhttp etcdserver/etcdhttp/httptypes pkg/fileutil pkg/flags pkg/idutil pkg/ioutil pkg/netutil pkg/osutil pkg/pbutil pkg/types pkg/transport pkg/wait proxy raft snap storage storage/backend store version wal"
 # TODO: add it to race testing when the issue is resolved
 # https://github.com/golang/go/issues/9946
 NO_RACE_TESTABLE="rafthttp"