Restructure test suite for basic upgrade tests

This tests upgrades by allowing the test cases that start NSO multiple times to use different versions. We still default to just testing with a single version of NSO, but this makes it possible to test with two different images and thus two different versions.

Notice how this doesn't incorporate multi-version a.k.a upgrade testing in CI. That will require some restructuring of the CI config. I think this is useful as it is and is easier to review as a change and thus keeping it separate.

A local test could be run with something like:

kll@minemacs:~/tailf/nso-docker$ make NSO_VERSION=4.7.5 build-version && make NSO_VERSION=5.3 build-version && make OLD_DOCKER_TAG=kll-4.7.5 NSO_VERSION=5.3 test-version

This first builds docker images out of version 4.7.6 and 5.3 and then attempts to run the test from 4.7.5 to 5.3. It will fail as we don't yet compact CDB which is required when upgrading from 4 to 5 (see #33 (closed)) but if run with two NSO 4.x versions or two 5.x versions it should work (I successfully tried 5.2.1 to 5.3).