Fix flaky mysqlctl blackbox test

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
==================
```

Signed-off-by: Dirkjan Bussink <[email protected]>

go/vt/logutil/logger.go

@@ -246,6 +246,12 @@ func (ml *MemoryLogger) Clear() {
 	ml.mu.Unlock()
 }
 
+func (ml *MemoryLogger) LogEvents() []*logutilpb.Event {
+	ml.mu.Lock()
+	defer ml.mu.Unlock()
+	return ml.Events
+}
+
 // LoggerWriter is an adapter that implements the io.Writer interface.
 type LoggerWriter struct {
 	logger Logger

go/vt/mysqlctl/blackbox/utils.go

@@ -78,7 +78,7 @@ func GetStats(stats *backupstats.FakeStats) StatSummary {
 
 func AssertLogs(t *testing.T, expectedLogs []string, logger *logutil.MemoryLogger) {
 	for _, log := range expectedLogs {
-		require.Truef(t, slices.ContainsFunc(logger.Events, func(event *logutilpb.Event) bool {
+		require.Truef(t, slices.ContainsFunc(logger.LogEvents(), func(event *logutilpb.Event) bool {
 			return event.GetValue() == log
 		}), "%s is missing from the logs", log)
 	}