DRILL-5783, DRILL-5841, DRILL-5894: Rationalize test temp directories
This change includes:
DRILL-5783:
- A unit test is created for the priority queue in the TopN operator.
- The code generation classes passed around a completely unused function registry reference in some places so it is removed.
- The priority queue had unused parameters for some of its methods so it is removed.
DRILL-5841:
- Created standardized temp directory classes DirTestWatcher, SubDirTestWatcher, and BaseDirTestWatcher. And updated all unit tests to use them.
DRILL-5894:
- Removed the dfs_test storage plugin for tests and replaced it with the already existing dfs storage plugin.
Misc:
- General code cleanup.
- Removed unnecessary use of String.format in the tests.
This closes #984
