Add a minimal example of using package:test.

[matanlurey]

Work towards flutter/flutter#133569.

This PR is a proof of concept that shows we're able to use package:test in flutter/engine instead of package:litetest.

I think it also shows that, if we're going to continue to vend dependencies this way, we might want to re-think our strategy in terms of using pub as a management tool - it's quite unwieldy already. For example, here is every pubspec.yaml file in the repo:

$ find . -name 'pubspec.yaml' -exec sh -c 'echo "$0 $(wc -l < "$0")"' {} \;

# Some files omitted in third_party or similar.
./impeller/tessellator/dart/pubspec.yaml       11
./tools/const_finder/pubspec.yaml       35
./tools/api_check/pubspec.yaml       90
./tools/build_bucket_golden_scraper/pubspec.yaml       47
./tools/licenses/pubspec.yaml       53
./tools/path_ops/dart/pubspec.yaml       26
./tools/engine_tool/pubspec.yaml       76
./tools/dir_contents_diff/pubspec.yaml       19
./tools/compare_goldens/pubspec.yaml        3
./tools/golden_tests_harvester/pubspec.yaml       55
./tools/gen_web_locale_keymap/pubspec.yaml       37
./tools/githooks/pubspec.yaml       63
./tools/android_lint/pubspec.yaml       35
./tools/clang_tidy/pubspec.yaml       76
./tools/pkg/engine_repo_tools/pubspec.yaml       41
./tools/pkg/process_fakes/pubspec.yaml       36
./tools/pkg/engine_build_configs/pubspec.yaml       73
./tools/pkg/git_repo_tools/pubspec.yaml       60
./tools/header_guard_check/pubspec.yaml       70
./sky/packages/sky_engine/pubspec.yaml        8
./shell/vmservice/pubspec.yaml        8
./ci/pubspec.yaml       57
./testing/benchmark/pubspec.yaml       77
./testing/skia_gold_client/pubspec.yaml       66
./testing/pkg_test_demo/pubspec.yaml      116
./testing/smoke_test_failure/pubspec.yaml       31
./testing/dart/pubspec.yaml       71
./testing/android_background_image/pubspec.yaml       22
./testing/litetest/pubspec.yaml       33
./testing/symbols/pubspec.yaml       24
./testing/scenario_app/pubspec.yaml       67
./web_sdk/web_engine_tester/pubspec.yaml       14
./web_sdk/web_test_utils/pubspec.yaml       22
./web_sdk/pubspec.yaml       60
./lib/snapshot/pubspec.yaml        8
./lib/gpu/pubspec.yaml       14
./lib/web_ui/pubspec.yaml       60
./flutter_frontend_server/pubspec.yaml       39

I'll file a follow-up issue to discuss pub-package management in the engine.

[zanderso]

nit: Needs the engine copyright header.

(Hopefully when we finish the buildmoot, this can be enforced by a tool.)

[matanlurey]

Done.

[zanderso]

Why are these excluded?

[matanlurey]

Shoot, this leaked in from another PR (the multi-pub one). Reverting.

[matanlurey]

(I actually think it makes sense to exclude them still, but let's do it in a standalone PR so I can explain why)