Use black30 for CupertinoTabBar's border (#119509)

Author: Sunbreak

packages/flutter/lib/src/cupertino/bottom_tab_bar.dart

@@ -15,7 +15,7 @@ import 'theme.dart';
 const double _kTabBarHeight = 50.0;
 
 const Color _kDefaultTabBarBorderColor = CupertinoDynamicColor.withBrightness(
-  color: Color(0x4C000000),
+  color: Color(0x4D000000),
   darkColor: Color(0x29000000),
 );
 const Color _kDefaultTabBarInactiveColor = CupertinoColors.inactiveGray;

packages/flutter/test/cupertino/bottom_tab_bar_test.dart

@@ -152,7 +152,7 @@ Future<void> main() async {
 
     // Border color is resolved correctly.
     final BoxDecoration decoration1 = renderDecoratedBox.decoration as BoxDecoration;
-    expect(decoration1.border!.top.color.value, 0x4C000000);
+    expect(decoration1.border!.top.color.value, 0x4D000000);
 
     // Switch to dark mode.
     await pumpWidgetWithBoilerplate(tester, MediaQuery(