[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. The gitlab-ci file is also renamed to its proper name, and the
non-passing, previously disabled "make:generic" test is commented out.
For the record, this 'build' test is strictly contained in the integration
1 job for integration-tests in 6 seconds (queued for 2 seconds)