Add a retry for setting the cookie in webview tests.

[freakboy3742]

#3068 added an API to retrieve cookies from WebView; however, the test for this feature has been flaky on iOS and macOS. This appears to be due to a race condition in the testbed - the on_load signal isn't a 100% reliable mechanism to prove that the webview is fully initialised and ready to accept Javascript calls.

You can overcome this by adding a short delay before setting the cookie. This PR adds a retry strategy to setting the cookie so that the delay is as short as is practical.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

Still intermittently failing on iOS: https://github.com/beeware/toga/actions/runs/12777096862/job/35617329195

[freakboy3742]

That's definitely the same test failing, but the manifestation is quite different.

Most notably, the exception that has surfaced is a TimeoutError - which isn't something that the testbed has raised directly. It's also notable that the cookie has only been retried 3 times (or, at least, the log message has only been printed 3 times) rather than 5.

If this was the same error, I'd expect to see 5 retries, then a "Cookie was not set" exception from L366. This stack trace looks more like the cookie test has passed after 3 tests, but then the test cleanup mechanism has failed for some reason. That's still a bug that needs to be fixed - but it's a different bug, and could be one that has been caused by some other state issue on the test machine. We'll have to see how often this new manifestation occurs.