test: add unit test cases

Author: rafamizes

test/brands/flutter_test.dart

@@ -24,5 +24,8 @@ void main() {
     test('light grey', () {
       expect(const Flutter.lightGrey().color, const Color(0xFFD5D7DA));
     });
+    test('white', () {
+      expect(const Flutter.white().color, const Color(0xFFFFFFFF));
+    });
   });
 }

test/brands/twitter_test.dart

@@ -27,5 +27,8 @@ void main() {
         const Color(0xFFF5F8FA),
       );
     });
+    test('white', () {
+      expect(const Twitter.white().color, const Color(0xFFFFFFFF));
+    });
   });
 }