Add integration tests for cloning GitLab repositories

[nathanwn]

We need some integration tests for cloning GitLab repositories. The following cases should be covered:

• Cloning a public repository from the public GitLab service, without an access token.
• Cloning a private repository from the public GitLab service, with an access token.
• Cloning a repository from a private GitLab service, with an access token.

Managing the access token in the GitHub Actions environment is one potential technical problem to consider.

Related issues: #316.