test: add unit tests for typography text styles

test/typography/text_styles_test.dart

@@ -0,0 +1,36 @@
+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:sudoku/typography/typography.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+  HttpOverrides.global = null;
+
+  group('SudokuTextStyle', () {
+    test('headlines are defined', () {
+      expect(SudokuTextStyle.headline1, isA<TextStyle>());
+      expect(SudokuTextStyle.headline2, isA<TextStyle>());
+      expect(SudokuTextStyle.headline3, isA<TextStyle>());
+      expect(SudokuTextStyle.headline4, isA<TextStyle>());
+      expect(SudokuTextStyle.headline5, isA<TextStyle>());
+      expect(SudokuTextStyle.headline6, isA<TextStyle>());
+    });
+
+    test('subtitles are defined', () {
+      expect(SudokuTextStyle.subtitle1, isA<TextStyle>());
+      expect(SudokuTextStyle.subtitle2, isA<TextStyle>());
+    });
+
+    test('body text are defined', () {
+      expect(SudokuTextStyle.bodyText1, isA<TextStyle>());
+      expect(SudokuTextStyle.bodyText2, isA<TextStyle>());
+    });
+
+    test('caption and button are defined', () {
+      expect(SudokuTextStyle.caption, isA<TextStyle>());
+      expect(SudokuTextStyle.button, isA<TextStyle>());
+    });
+  });
+}