COCKROACH_BINARY does not work as expected

COCKROACH_BINARY does not work with "~" paths, and is relative to the `testing` folder (as opposed to the folder with the Makefile), which should be called out. While on the subject of documentation, we should make `make dockertest` a first class citizen, it's much better than asking someone looking to jump in to figure out all the ORM dependencies they need to install.

We should include a `COCKROACH_BINARY` usage example for both `make test` and `make dockertest` (which would rely on a linux build of CRDB).