There are needs to simulate sync heavy work, such as a slow widget build, in flutter widget tests. This method simply expose that.

As a remark, this cannot be replaced by runAsync. Consider the following example:

    testWidgets('can use to simulate slow build', (WidgetTester tester) async {
      final DateTime beforeTime = binding.clock.now();

      await tester.pumpWidget(Builder(builder: (_) {
        bool timerCalled = false;
        Timer.run(() => timerCalled = true);

        binding.elapseBlocking(const Duration(seconds: 1));

        // if we use `delayed` instead of `elapseBlocking`, such as
        // binding.delayed(const Duration(seconds: 1));
        // the timer will be called here. Surely, that violates how
        // a flutter widget build works
        expect(timerCalled, false);

        return Container();
      }));

      expect(binding.clock.now(), beforeTime.add(const Duration(seconds: 1)));
      binding.idle();
    });

As is discussed in the comments in the example, if we use delayed, timers will be fired when executing half of a build function, which is totally wrong.

List which issues are fixed by this PR. You must list at least one issue.
Close #112620

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Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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