Look into memory bloat after running flutter test

While running
`flutter test test/`
from `packages/devtools_app`
about 40 times, and realized that my cloudtop was running out of memory.
I noticed that `devtools/packages/devtools_app/test/fixtures` had grown to 34GB

it may be worth exploring why that memory is being left behind. Kenzie had the initial idea that perhaps some test fixtures aren't properly being torn down

It looks like running `flutter clean` deletes the `.dart_tool` directory, which is the main culprit for where these files are left over.
So finding a way to document or have that happen automatically may be helpful

`packages/devtools_app` から `flutter test test/` を実行して増加を再現し、その後 `test/fixtures` と関連する `.dart_tool` ファイルを調べます。`flutter clean` の前後の状態を比較し、fixture データが残されるかどうかを明らかにしたうえで、クリーンアップの動作を文書化するか自動化することを完了条件とします。

索引モデルが issue の本文から書いたものです。