Fix a ConstantTween's clerical error and Add some Tween testcases. (#71830)

packages/flutter/lib/src/animation/tween.dart

@@ -436,7 +436,7 @@ class ConstantTween<T> extends Tween<T> {
   T lerp(double t) => begin as T;
 
   @override
-  String toString() => '${objectRuntimeType(this, 'ReverseTween')}(value: $begin)';
+  String toString() => '${objectRuntimeType(this, 'ConstantTween')}(value: $begin)';
 }
 
 /// Transforms the value of the given animation by the given curve.

packages/flutter/test/animation/tween_test.dart

@@ -93,4 +93,34 @@ void main() {
     expect(tween.lerp(0.5), 100.0);
     expect(tween.lerp(1.0), 100.0);
   });
+
+  test('ReverseTween', () {
+    final ReverseTween<int> tween = ReverseTween<int>(IntTween(begin: 5, end: 9));
+    expect(tween.lerp(0.5), 7);
+    expect(tween.lerp(0.7), 6);
+  });
+
+  test('ColorTween', () {
+    final ColorTween tween = ColorTween(
+      begin: const Color(0xff000000),
+      end: const Color(0xffffffff)
+    );
+    expect(tween.lerp(0.0), const Color(0xff000000));
+    expect(tween.lerp(0.5), const Color(0xff7f7f7f));
+    expect(tween.lerp(0.7), const Color(0xffb2b2b2));
+    expect(tween.lerp(1.0), const Color(0xffffffff));
+  });
+
+  test('StepTween', () {
+    final StepTween tween = StepTween(begin: 5, end: 9);
+    expect(tween.lerp(0.5), 7);
+    expect(tween.lerp(0.7), 7);
+  });
+
+  test('CurveTween', () {
+    final CurveTween tween = CurveTween(curve: Curves.easeIn);
+    expect(tween.transform(0.0), 0.0);
+    expect(tween.transform(0.5), 0.31640625);
+    expect(tween.transform(1.0), 1.0);
+  });
 }