br: use random port to start mock cluster.

[3pointer]

What problem does this PR solve?

fix unstable ut in br pkg.

What is changed and how it works?

1. remove multiple mock.Start()/mock.Stop() in restore test.
2. use net.Listen() to get a random port.

Check List

Tests

• Unit test

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• YuJuncen
• fengou1

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/01968e69352305180a4a9e5e278763f20f5d3a5a

[3pointer]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: e7e1d52

[ti-chi-bot]

[FORMAT CHECKER NOTIFICATION]

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[3pointer]

/run-check_dev_2