br/pkg/backup: migrate test-infra to testify

[tisonkun]

Signed-off-by: tison wander4096@gmail.com

What problem does this PR solve?

Issue Number: close #28164

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hawkingrei
• tangenta

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[tisonkun]

/cc @hawkingrei @glorv

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/751f204dd8b07efce16b1ce001e1964ce83f4791

[tisonkun]

ping @glorv @hawkingrei @YuJuncen

[tisonkun]

/cc @YuJuncen

[hawkingrei]

@tangenta PTAL

[tangenta]

/merge

[ti-chi-bot]

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

Commit hash: ed2bfc4

[ti-chi-bot]

@tisonkun: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

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