[webview_flutter] Migrate more integration tests to local web server

[stuartmorgan-g]

Replaces youtube.com URLs in integration tests with URLs using the local web server
running inside of the integration test.

No CHANGELOG change: Only internal test details are changing

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 relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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.

[stuartmorgan-g]

AFAICT the issue here is that the webview delegate is sometimes (it doesn't happen for me locally) using the delegate from the example, not the test, which suggest there's some kind of race condition and state bleed here. I have no idea why that would be happening though.

[stuartmorgan-g]

I've split video_player changes into #5103 to unblock those.

[stuartmorgan-g]

Closing until I have time to revisit this and figure out what's going on with the delegate.