Testable and Immutability of ObjectBox

Request

Make ObjectBox object is suitable to unit tests

Override Equality of Condition and implementations of Condition

Thus following code can work

       verify(() => mockBox.query(Entity_.id.equals(id))).called(1);

and use const constructor for ObjectBoxException

/// ObjectBox database exception.
class ObjectBoxException implements Exception {
  /// Exception message.
  final String message;

  /// Create a new exception.
  ObjectBoxException(this.message);  // <--- add const

  @override
  String toString() => 'ObjectBoxException: $message';
}