br: migrate test-infra to testify for br/pkg/task/backup_test.go

[tongtongyin]

What problem does this PR solve?

Issue Number: close #27178

Problem Summary:

What is changed and how it works?

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

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[tongtongyin]

@tisonkun

[tisonkun]

This PR cannot close tracking issue #27178. Please specify the dedicated sub-issue.

Also, the diff is invalid. You cannot change only one file and remove TestingT while there are other testing files.

Please follow other PRs under this effort first.