Simplistic Calculator (#1030)

* Simplistic Calculator, initial commit

* Add Linux runner

* Add macOS runner, update `main.dart` for iOS

* Add a test

simplistic_calculator/test/widget_test.dart

@@ -0,0 +1,36 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:simplistic_calculator/main.dart';
+
+void main() {
+  testWidgets('Calculator smoke test', (tester) async {
+    // Build our app and trigger a frame.
+    await tester.pumpWidget(
+      const ProviderScope(
+        child: CalculatorApp(),
+      ),
+    );
+
+    // Verify that our counter starts at 1 through 9, + and =.
+    expect(find.text('1'), findsOneWidget);
+    expect(find.text('2'), findsOneWidget);
+    expect(find.text('3'), findsOneWidget);
+    expect(find.text('4'), findsOneWidget);
+    expect(find.text('5'), findsOneWidget);
+    expect(find.text('6'), findsOneWidget);
+    expect(find.text('7'), findsOneWidget);
+    expect(find.text('8'), findsOneWidget);
+    expect(find.text('9'), findsOneWidget);
+    expect(find.text('+'), findsOneWidget);
+    expect(find.text('='), findsOneWidget);
+
+    await tester.tap(find.text('9'));
+    await tester.tap(find.text('+'));
+    await tester.tap(find.text('6'));
+    await tester.tap(find.text('='));
+    await tester.pump();
+
+    // Verify that our calculator evaluates correctly.
+    expect(find.text('15'), findsOneWidget);
+  });
+}