[webview_flutter] Fix debuggingEnabled on Android

[stuartmorgan-g]

The recent Dart rewrite accidentally dropped a parameter value in the
call chain, causing disabling debugging to enabled it instead.

This fixes the issue, and adds testing of that portion of the call
chain (which was entirely missing), as well as adding testing of false
at various other points just for added coverage.

As opportunistic cleanup, changes the test group naming, as the pattern
of using '$FooClass' breaks test result display in VSCode.

Fixes flutter/flutter#98521

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• 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.

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[stuartmorgan-g]

We should look into re-running code coverage analysis of this, and (before switching to it) running it on the new iOS implementation, and look for gaps like this class being completely untested. The downside of this new structure is that there are more layers where a bug like this can sneak in, and coverage analysis will help catch things like this in advance.

(Although in this case all the tests of the rest of the chain of calls also only tested true, so coverage wouldn't have found that anyway. But at least it would have flagged the need to have tests of this object.)

[bparrishMines]

LGTM