backup: skip TestBackupExportRequestTimeout under deadlock

Informs #154840

Release note: none

Author: msbutler

pkg/backup/backup_test.go

@@ -7602,6 +7602,8 @@ func TestBackupExportRequestTimeout(t *testing.T) {
 	defer leaktest.AfterTest(t)()
 	defer log.Scope(t).Close(t)
 
+	skip.UnderDeadlock(t)
+
 	allowRequest := make(chan struct{})
 	defer close(allowRequest)