Fix flaky mysqlctl blackbox test

[dbussink]

This can trigger the race detector:

==================
WARNING: DATA RACE
Read at 0x00c00031f900 by goroutine 64:
  vitess.io/vitess/go/vt/mysqlctl/blackbox.AssertLogs()
      /Users/dirkjan/code/vitessio/vitess/go/vt/mysqlctl/blackbox/utils.go:81 +0xb8
  vitess.io/vitess/go/vt/mysqlctl/blackbox.TestExecuteBackupFailToWriteFileTwice()
      /Users/dirkjan/code/vitessio/vitess/go/vt/mysqlctl/blackbox/backup_test.go:713 +0x7e8
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:1690 +0x184
  testing.(*T).Run.gowrap1()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:1743 +0x40

Previous write at 0x00c00031f900 by goroutine 75:
  vitess.io/vitess/go/vt/mysqlctl/blackbox.TestExecuteBackupFailToWriteFileTwice.NewMemoryLogger.func2()
      /Users/dirkjan/code/vitessio/vitess/go/vt/logutil/logger.go:225 +0x130
  vitess.io/vitess/go/vt/logutil.(*CallbackLogger).InfoDepth()
      /Users/dirkjan/code/vitessio/vitess/go/vt/logutil/logger.go:139 +0x1a0
  vitess.io/vitess/go/vt/logutil.(*CallbackLogger).Infof()
      /Users/dirkjan/code/vitessio/vitess/go/vt/logutil/logger.go:174 +0x60
  vitess.io/vitess/go/vt/logutil.(*MemoryLogger).Infof()
      <autogenerated>:1 +0x5c
  vitess.io/vitess/go/vt/mysqlctl.(*backupPipe).ReportProgress()
      /Users/dirkjan/code/vitessio/vitess/go/vt/mysqlctl/builtinbackupengine.go:820 +0xac4
  vitess.io/vitess/go/vt/mysqlctl.(*BuiltinBackupEngine).backupFile.gowrap1()
      /Users/dirkjan/code/vitessio/vitess/go/vt/mysqlctl/builtinbackupengine.go:869 +0x90

Goroutine 64 (running) created at:
  testing.(*T).Run()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:1743 +0x5e0
  testing.runTests.func1()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:2168 +0x80
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:1690 +0x184
  testing.runTests()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:2166 +0x6e0
  testing.(*M).Run()
      /opt/homebrew/Cellar/go/1.23.4/libexec/src/testing/testing.go:2034 +0xb74
  main.main()
      _testmain.go:59 +0x110

Goroutine 75 (finished) created at:
  vitess.io/vitess/go/vt/mysqlctl.(*BuiltinBackupEngine).backupFile()
      /Users/dirkjan/code/vitessio/vitess/go/vt/mysqlctl/builtinbackupengine.go:869 +0x734
  vitess.io/vitess/go/vt/mysqlctl.(*BuiltinBackupEngine).backupFileEntries.func2()
      /Users/dirkjan/code/vitessio/vitess/go/vt/mysqlctl/builtinbackupengine.go:706 +0x294
  golang.org/x/sync/errgroup.(*Group).Go.func1()
      /Users/dirkjan/go/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:78 +0x7c
==================

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

• Apply the release notes (needs details) label if users need to know about this change.
• New features should be documented.
• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

New flags

• Is this flag really necessary?
• Flag names must be clear and intuitive, use dashes (-), and have a clear help text.

If a workflow is added or modified:

• Each item in Jobs should be named in order to mark it as required.
• If the workflow needs to be marked as required, the maintainer team must be notified.

Backward compatibility

• Protobuf changes should be wire-compatible.
• Changes to _vt tables and RPCs need to be backward compatible.
• RPC changes should be compatible with vitess-operator
• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• vtctl command output order should be stable and awk-able.

[mattlord]

Nit, but IMO GetEvents() is a better name. If it's not safe to access Events directly, then it may also make sense to make it private (rename it to events).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.51%. Comparing base (44aa9d7) to head (bf51e68).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17387      +/-   ##
==========================================
+ Coverage   67.47%   67.51%   +0.04%     
==========================================
  Files        1581     1581              
  Lines      253944   253948       +4     
==========================================
+ Hits       171353   171464     +111     
+ Misses      82591    82484     -107     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.