Add integration tests for gitlabktl

Description

We should add integration tests for gitlabktl into this repository.

We could basically do gitlabktl serverless build and gitlabktl serverless deploy here, in this repository.

These are not end-to-end tests because we are not testing GitLab - Knative integration here, we are only testing gitlabktl tool separately.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information