Add a new dt presubmit tool.

[kenzieschmoll]

This presubmit command mimics the presubmits that are run on the CI. Optionally, when a --fix flag is passed, the command applies dart fixes and formatting changes before performing the presubmit checks. Order of operations:

1. Runs pub get to ensure packages solve and are up to date
2. (when the --fix flag is passed) Runs dart fix --apply and dart format to fix any warnings with associated dart fixes and address formatting issues
3. Runs dt repo-check
4. Runs dt analyze
5. (when the --fix flag is not passed) Runs dart format over the DevTools packages exits with a failure code if anything changed.

This also adds utilities to test dt tool commands.

Work towards #9724.