This patch fixes a build/test issue caused by the combination of #10672 and a latent issue in the original `dev/test-dependencies` script.

First, changes which _only_ touched build files were not triggering full Jenkins runs, making it possible for a build change to be merged even though it could cause failures in other tests. The `root` build module now depends on `build`, so all tests will now be run whenever a build-related file is changed.

I also added a `clean` step to the Maven install step in `dev/test-dependencies` in order to address an issue where the dummy JARs stuck around and caused "multiple assembly JARs found" errors in tests.

/cc zsxwing

Author: Josh Rosen <joshrosen@databricks.com>

Closes #10704 from JoshRosen/fix-build-test-problems.