[ci] Added argotest-based integration tests

This commits adds a gitlab-ci integration test target, and the
.integration.nix file, which specifies the exact test version pinning to
use.
1 job for <span class="ref-name">integration-tests</span> in 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #16268
integration
integration.test

00:00:34