add CameraFacing test

+import 'package:flutter_test/flutter_test.dart';
+import 'package:mobile_scanner/src/enums/camera_facing.dart';
+
+void main() {
+  group('$CameraFacing tests', () {
+    test('can be created from raw value', () {
+      const values = <int, CameraFacing>{
+        0: CameraFacing.front,
+        1: CameraFacing.back,
+      };
+
+      for (final MapEntry<int, CameraFacing> entry in values.entries) {
+        final CameraFacing result = CameraFacing.fromRawValue(entry.key);
+
+        expect(result, entry.value);
+      }
+    });
+
+    test('invalid raw value throws assertion error', () {
+      const int negative = -1;
+      const int outOfRange = 2;
+
+      expect(() => CameraFacing.fromRawValue(negative), throwsArgumentError);
+      expect(() => CameraFacing.fromRawValue(outOfRange), throwsArgumentError);
+    });
+
+    test('can be converted to raw value', () {
+      const values = <CameraFacing, int>{
+        CameraFacing.front: 0,
+        CameraFacing.back: 1,
+      };
+
+      for (final MapEntry<CameraFacing, int> entry in values.entries) {
+        final int result = entry.key.rawValue;
+
+        expect(result, entry.value);
+      }
+    });
+  });
+}