router: fix unstable test TestConnBalanced

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:
TestConnBalanced is unstable because I forgot to call redirectFinish before checking metrics.

What is changed and how it works:
Add redirectFinish. I tested it several times and it's OK now.

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 (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None