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