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.