dev/ci: restructure integration tests, unify test server setup

Created by: bobheadxi

• unify sourcegraph/server setup in integration tests so that we use the same scripts in each
• move all integration tests into new dev/ci/integration directory, and introduce a structure of run.sh and test.sh - see dev/ci/integration/README.md for overview
• some minor refactors to unify the tests a bit more
• remove unused servers.yaml that was part of the Vagrant setup (see https://github.com/sourcegraph/sourcegraph/issues/28709)

Part of https://github.com/sourcegraph/sourcegraph/issues/29413 and maybe https://github.com/sourcegraph/sourcegraph/issues/26182 - this work makes it easier to switch up the underlying Sourcegraph instance used in testing.