Fix a ConstantTween's clerical error and Add some Tween testcases.

[feHouyushan]

Description

The ReverseTween in "ConstantTween.toString" might be a clerical error here, right?

I think it may be ConstantTween.

/// A tween with a constant value.
class ConstantTween<T> extends Tween<T> {
  /// Create a tween whose [begin] and [end] values equal [value].
  ConstantTween(T value) : super(begin: value, end: value);

  /// This tween doesn't interpolate, it always returns the same value.
  @override
  T lerp(double t) => begin as T;

  @override
  String toString() => '${objectRuntimeType(this, 'ReverseTween')}(value: $begin)';
}

I fixed it and Added some Tween testcases.

Tests

I added the following tests:

test/animation/tween_test.dart

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[goderbauer]

LGTM