Does a few things:
- Refactors `run_command_test` significantly to reduce global fixtures
- Replaced stringly-typed things with enum-like objects
- Adds a lot stronger coverage for `run_command` to make future refactors safer
- Takes advantage of `package:test` having a workable matchers system and uses it
- Changes `return 1` into `throw FatalError(...)` where it makes sense in `run_command`
As a result of the refactoring work, I also fixed a bug: Closes https://github.com/flutter/flutter/issues/147646.
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