qa: automated sourcegraph/server upgrade test

Created by: slimsag

Today our release process https://github.com/sourcegraph/sourcegraph/issues/13374 indicates we should do a manual test of upgrading Sourcegraph using the server image:

• Upgrade from previous release:
  • Run the initializer on a Docker container running the last patch version of the previous major/minor release.
  • Upgrade and run the regression test suite.

We should effectively copy what we did for https://github.com/sourcegraph/deploy-sourcegraph-docker/tree/master/test and have a Vagrant-based test for this that runs in CI.

One challenge here will be figuring out how to denote the "old" and "new" version - will probably have to fetch that info from Git tags?