[SPARK-13023][PROJECT INFRA] Fix handling of root module in modules_to_test()

There's a minor bug in how we handle the `root` module in the `modules_to_test()` function in `dev/run-tests.py`: since `root` now depends on `build` (since every test needs to run on any build test), we now need to check for the presence of root in `modules_to_test` instead of `changed_modules`.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #10933 from JoshRosen/build-module-fix.