add MobileScannerState test

+import 'package:flutter_test/flutter_test.dart';
+import 'package:mobile_scanner/src/enums/mobile_scanner_state.dart';
+
+void main() {
+  group('$MobileScannerState tests', () {
+    test('can be created from raw value', () {
+      const values = {
+        0: MobileScannerState.undetermined,
+        1: MobileScannerState.authorized,
+        2: MobileScannerState.denied,
+      };
+
+      for (final MapEntry<int, MobileScannerState> entry in values.entries) {
+        final MobileScannerState result = MobileScannerState.fromRawValue(entry.key);
+
+        expect(result, entry.value);
+      }
+    });
+
+    test('invalid raw value throws argument error', () {
+      const int negative = -1;
+      const int outOfRange = 3;
+
+      expect(() => MobileScannerState.fromRawValue(negative), throwsArgumentError);
+      expect(() => MobileScannerState.fromRawValue(outOfRange), throwsArgumentError);
+    });
+
+    test('can be converted to raw value', () {
+      const values = <MobileScannerState, int>{
+        MobileScannerState.undetermined: 0,
+        MobileScannerState.authorized: 1,
+        MobileScannerState.denied: 2,
+      };
+
+      for (final MapEntry<MobileScannerState, int> entry in values.entries) {
+        final int result = entry.key.rawValue;
+
+        expect(result, entry.value);
+      }
+    });
+  });
+}