Update BottomNavigationBar tests for M3 (#136624)

This PR updates `BottomNavigationBar` unit tests for M3 migration.

More info in https://github.com/flutter/flutter/issues/127064

It was somewhat complex because existing tests relied on a lot of magic numbers.

packages/flutter/lib/src/material/bottom_navigation_bar.dart

@@ -1159,7 +1159,7 @@ class _BottomNavigationBarState extends State<BottomNavigationBar> with TickerPr
                   removeBottom: true,
                   child: DefaultTextStyle.merge(
                     overflow: TextOverflow.ellipsis,
-                    child:  Row(
+                    child: Row(
                       mainAxisAlignment: MainAxisAlignment.spaceBetween,
                       children: _createTiles(layout),
                     ),