COCKROACH_BINARY does not work as expected

[irfansharif]

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).

[irfansharif]

I'm told the path issue is an underlying problem in https://github.com/cockroachdb/cockroach-go/blob/master/testserver/testserver.go, but this is where I ran into it, so 🏃