[REVIEW][Java] Refactor CagraBuildAndSearchIT to explicitly express different execution modes (rapidsai#1006)

This PR refactors `CagraBuildAndSearchIT`, introducing separate test for different execution modes (same thread, different thread, concurrent).
This way different modes are tested every time, without the need to rely on randomization to hit a particular test path.
The execution is handled by utility functions (e.g. `runConcurrently`); right now these are private to the test suite, but if we think they could be useful across other tests I can move them to a utility class.

It also fixes a issue where we did not wait and/or report failures in the concurrent case (now exceptions are propagated back to the calling JUnit thread).

Authors:
  - Lorenzo Dematté (https://github.com/ldematte)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Chris Hegarty (https://github.com/ChrisHegarty)
  - MithunR (https://github.com/mythrocks)

URL: rapidsai#1006

Author: ldematte