[webview_flutter] Fix Flaky Resize Test

[bparrishMines]

Split of #4455

After reading through https://flutter.dev/docs/cookbook/testing/widget/introduction. It seems that this test isn't using a Widget test in the expected way. After more testing, the current solution does work in updating the widget, but still has a false positive for the 2nd reason below.

1. It's attempting to change the size of a WebView by pumping a new Widget.
2. I was able to make this test fail by adding a await Future.delayed(Duration(seconds: 2)); before the boolean check. This shows that resize is called regardless of changing the width and height.

This PR follows the Flutter cookbook example of a Widget test and creates a widget that changes the WebView on a button press. It also waits for the initial resize of the WebView from the initial load.

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[bparrishMines]

This is required to wait for the initial resize of the WebView. The line await pageFinishedCompleter.future; can finish before this.

[stuartmorgan-g]

Please capture this in a code comment, as it's non-obvious.

[stuartmorgan-g]

LGTM with nits.

(Related though: do we need three copies of this test? It's not clear to me where exactly the functionality that it's intended to test lives; are there important parts that live in both native an app-facing-package-dart?)

[stuartmorgan-g]

Please capture this in a code comment, as it's non-obvious.

[stuartmorgan-g]

How were the 3-second values chosen? It seems like a very arbitrary number.

[bparrishMines]

It is mostly arbitrarily chosen :/. I just chose a safe value to wait for. I considered just waiting for the first call to resize, but I wasn't 100% sure it gets called every time on every platform.

[stuartmorgan-g]

Wait, so it this always waiting 3 seconds? I'm not all that familiar with widget tests; I assumed this was a timeout. If it's a static 3 second wait, that's still going to be a source of flake.

[bparrishMines]

That's a good point. I changed the tests to wait for resize completers.

[stuartmorgan-g]

LGTM