unstable test case TestAnalyzeIndexExtractTopN

[XuHuaiyu]

https://internal.pingcap.net/idc-jenkins/blue/organizations/jenkins/tidb_ghpr_unit_test/detail/tidb_ghpr_unit_test/71046/pipeline/82

[2021-04-14T04:14:42.805Z] FAIL: analyze_test.go:238: testSuite1.TestAnalyzeIndexExtractTopN
[2021-04-14T04:14:42.805Z] 
[2021-04-14T04:14:42.805Z] analyze_test.go:288:
[2021-04-14T04:14:42.805Z]     c.Assert(idx.CMSketch.Equal(cms), IsTrue)
[2021-04-14T04:14:42.805Z] ... obtained bool = false

[XuHuaiyu]

skip this test in PR #24003

[cosven]

/severity major
/sig planner

[qw4990]

Close since this failure didn't appear for a few weeks according to our monitoring.
If it appear again, please reopen it and mention me in this issue.

[ti-srebot]

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[qw4990]

My mistake. This test is skipped now, so this problem may still exist.
Since this problem only affect CI, I degrade its severity to moderate and I will fix it soon.