Mark unusuallyLongTimeout as internal (#68038)

packages/flutter_driver/lib/src/driver/driver.dart

diff --git a/packages/flutter_driver/lib/src/driver/driver.dart b/packages/flutter_driver/lib/src/driver/driver.dart
index 8b379ec..1703b5e 100644
--- a/packages/flutter_driver/lib/src/driver/driver.dart
+++ b/packages/flutter_driver/lib/src/driver/driver.dart
@@ -67,7 +67,7 @@
 
 /// How long to wait before showing a message saying that
 /// things seem to be taking a long time.
-@visibleForTesting
+@internal
 const Duration kUnusuallyLongTimeout = Duration(seconds: 5);
 
 /// A convenient accessor to frequently used finders.