backend, infosync: optimize TestHandlerReturnError and TestEtcdServerDown4Sync

[djshow832]

What problem does this PR solve?

Issue Number: close #374

Problem Summary:

• TestHandlerReturnError takes 15s
• TestEtcdServerDown4Sync takes 17s

What is changed and how it works:

• Add ConnectTime to BCConfig so I can make the timeout shorter in TestHandlerReturnError. I prefer not to put it into configuration because connection timeout barely happens in self-hosted deployment.
• Reduce loops from 5 to 1 for TestEtcdServerDown4Sync.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Notable changes

• Has configuration change
• Has HTTP API interfaces change
• Has tiproxyctl change
• Other user behavior changes

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: xhebox

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-11-13 02:32:19.730376111 +0000 UTC m=+4043537.317486241: ☑️ agreed by xhebox.