flaky test TestRewriteRules

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

    batcher_test.go:326: 
        	Error Trace:	/home/jenkins/.tidb/tmp/63a9840cd0739f2c243bb46478607469/execroot/__main__/bazel-out/k8-fastbuild/bin/br/pkg/restore/restore_test_/restore_test.runfiles/__main__/br/pkg/restore/batcher_test.go:326
        	Error:      	Should be true
        	Test:       	TestRewriteRules

4. What is your TiDB version? (Required)