Tests: macOS now correctly throws unique violation exception.

Follow-up from
ccec6e8 unblock macOS CI failure

objectbox_test/test/box_test.dart

@@ -1,7 +1,5 @@
 // ignore_for_file: deprecated_member_use
 
-import 'dart:io';
-
 import 'package:objectbox/objectbox.dart';
 import 'package:test/test.dart';
 
@@ -240,18 +238,11 @@ void main() {
       final object = TestEntity2()..value = 42;
       final future = box.putQueuedAwaitResult(object);
 
-      try {
-        await future;
-      } catch (e) {
-        // TODO: Mac in GitHub CI (not locally reproducible yet)...
-        if (Platform.isMacOS) {
-          expect(e is ObjectBoxException, isTrue);
-          expect((e as ObjectBoxException).message, '');
-        } else {
-          expect(e is UniqueViolationException, isTrue);
-          expect(e.toString(), contains('Unique constraint'));
-        }
-      }
+      expect(
+          () async => await future,
+          throwsA(predicate((e) =>
+              e is UniqueViolationException &&
+              e.message.contains('Unique constraint'))));
 
       expect(object.id, isNull); // ID must remain unassigned
     }