Clean up flutter_test/test/controller_test.dart (#31333)

Instead of using a custom WidgetController, which is very brittle, we just use the usual infrastructure.

Also, use structured data instead of an array.

This adds offsetMoreOrLessEquals to handle small floating point errors in offsets.

packages/flutter_test/lib/src/matchers.dart

@@ -206,12 +206,14 @@ Matcher isInstanceOf<T>() => test_package.TypeMatcher<T>();
 
 /// Asserts that two [double]s are equal, within some tolerated error.
 ///
+/// {@template flutter.flutter_test.moreOrLessEquals.epsilon}
 /// Two values are considered equal if the difference between them is within
 /// [precisionErrorTolerance] of the larger one. This is an arbitrary value
 /// which can be adjusted using the `epsilon` argument. This matcher is intended
 /// to compare floating point numbers that are the result of different sequences
 /// of operations, such that they may have accumulated slightly different
 /// errors.
+/// {@endtemplate}
 ///
 /// See also:
 ///
@@ -219,28 +221,40 @@ Matcher isInstanceOf<T>() => test_package.TypeMatcher<T>();
 ///    required and not named.
 ///  * [inInclusiveRange], which matches if the argument is in a specified
 ///    range.
+///  * [rectMoreOrLessEquals] and [offsetMoreOrLessEquals], which do something
+///    similar but for [Rect]s and [Offset]s respectively.
 Matcher moreOrLessEquals(double value, { double epsilon = precisionErrorTolerance }) {
   return _MoreOrLessEquals(value, epsilon);
 }
 
 /// Asserts that two [Rect]s are equal, within some tolerated error.
 ///
-/// Two values are considered equal if the difference between them is within
-/// [precisionErrorTolerance] of the larger one. This is an arbitrary value
-/// which can be adjusted using the `epsilon` argument. This matcher is intended
-/// to compare floating point numbers that are the result of different sequences
-/// of operations, such that they may have accumulated slightly different
-/// errors.
+/// {@macro flutter.flutter_test.moreOrLessEquals.epsilon}
 ///
 /// See also:
 ///
 ///  * [moreOrLessEquals], which is for [double]s.
+///  * [offsetMoreOrLessEquals], which is for [Offset]s.
 ///  * [within], which offers a generic version of this functionality that can
 ///    be used to match [Rect]s as well as other types.
 Matcher rectMoreOrLessEquals(Rect value, { double epsilon = precisionErrorTolerance }) {
   return _IsWithinDistance<Rect>(_rectDistance, value, epsilon);
 }
 
+/// Asserts that two [Offset]s are equal, within some tolerated error.
+///
+/// {@macro flutter.flutter_test.moreOrLessEquals.epsilon}
+///
+/// See also:
+///
+///  * [moreOrLessEquals], which is for [double]s.
+///  * [rectMoreOrLessEquals], which is for [Rect]s.
+///  * [within], which offers a generic version of this functionality that can
+///    be used to match [Offset]s as well as other types.
+Matcher offsetMoreOrLessEquals(Offset value, { double epsilon = precisionErrorTolerance }) {
+  return _IsWithinDistance<Offset>(_offsetDistance, value, epsilon);
+}
+
 /// Asserts that two [String]s are equal after normalizing likely hash codes.
 ///
 /// A `#` followed by 5 hexadecimal digits is assumed to be a short hash code