VReplication Flaky Test fix: TestVtctldMigrateSharded

[rohit-nayak-ps]

Description

TestVtctldMigrateSharded has been flaky for a while: https://app.launchableinc.com/organizations/vitess/workspaces/vitess-app/data/test-paths/class%3Dvreplication%23%23%23testcase%3DTestVtctldMigrateSharded. However it runs with no issues locally even with a significant number of runs.

Currently there are a lot of tests in the same test shard, which might account for the flakiness. We will try by moving TestMigrate(Un)Sharded to their own CI test 'shard' and monitor its flakiness.

Related Issue(s)

Checklist

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[vitess-bot]

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