Reland "[data_assets] Cleanup tests" (#184714)

Relands https://github.com/flutter/flutter/pull/184209 adding a mac
shard as suggested in
https://github.com/flutter/flutter/pull/184209#issuecomment-4184741423.

For adding a shard, I followed
https://github.com/flutter/flutter/pull/154444, which suggests it's
contained in the single `.ci.yaml` file, and the new shard names need to
have `bringup: true`.


----

Original PR description:

This cleans up the data_assets experiment testing.

* Use integration test projects, which are statically analyzed,
formatted, etc.
* This forces us to be explicit about the `package:data_assets`
dependency.
* This means the test projects will roll their dependencies with the
dependency auto roller. (The test project was still using
`native_assets_cli` which has been replaced with `hooks` and
`data_assets`.)
* Pin `package:data_assets`, because it's experimental, and we might do
breaking changes.

Also makes the test projects more useful:

* The `data_assets_app` now displays the assets in the UI. (Not only
prints to the console for the integration tests.)
* The `data_assets_package` now actually uses its asset.

And cleans up the test projects:

* Move the test files into the `data/` dir.
* Give the test data assets a proper extension.

I saw weird behavior on Windows:

* DevFS doesn't pick up the writes to the Dart files, not even with
flushing or sleeps: `Compiling dart to kernel with 0 updated files`
leading to a state where the Dart variable contents don't get updated
while the data assets do. This integration test is not really about Dart
hot restart, so I've skipped those checks on Windows.
* This reproduces locally in the integration test, but not when running
the app and editing the file.

The checks in dev/bots/analyze.dart also required some updates:

* The template files check didn't allow for the HTML license.

P.S. Sorry for all the generated Flutter scaffolding. It seems that that
is the way it's done in the integration test directory. (The package
with the data assets is clean, but the Flutter app ...)