Add et run command (#51037)

The `run` command builds both the host and target engines and then invokes `flutter run` so that it runs the app using the custom engine builds.

It is expected that 'et run' be used inside the directory of a flutter application.

Command line flags passed after `--` will be forwarded to `flutter run`.

Some examples:

- `et run` Build the debug variant and runs the app in that mode.
- `et run -- --profile` Build the profile variant and runs the app in that mode.
- `et run -- --release` Build the release variant and runs the app in that mode.

Also:

- Start a local_engine.json builder definition (it is missing a lot)
- Tweak the test fixture.
- Add a new Matcher to litetest.
- Tweaked the build config linter to only care about duplicate build names within the same builder.