unstable unit test TestScaleNode

[wk989898]

What did you do?

https://prow.tidb.net/view/gs/prow-tidb-logs/pr-logs/pull/pingcap_ticdc/4809/pull-unit-test/2044689022244098048

What did you expect to see?

No response

What did you see instead?

    coordinator_test.go:482: 
        	Error Trace:	/home/prow/go/src/github.com/pingcap/ticdc/coordinator/coordinator_test.go:482
        	Error:      	Condition never satisfied
        	Test:       	TestScaleNode

Versions of the cluster

Upstream TiDB cluster version (execute SELECT tidb_version(); in a MySQL client):

(paste TiDB cluster version here)

Upstream TiKV version (execute tikv-server --version):

(paste TiKV version here)

TiCDC version (execute cdc version):

(paste TiCDC version here)