test: attempt to make functional test startup more reliable

The functional tests have been erratic. Current theory is that we are
starting the tests before the GitLab container is fully up and
running.

  * Add checking of the Health Check[1] endpoints.
  * Add a 20 second delay after we believe it is up and running.
  * Increase timeout from 300 to 400 seconds

[1] https://docs.gitlab.com/ee/user/admin_area/monitoring/health_check.html

Use the GitLab docker image 15.2.0-ee.0 in the functional testing.

This reverts commit 67ab24fe5ae10a9f8cc9122b1a08848e8927635d.

Enable using GitLab Enterprise Edition (EE) in the functional tests.
This will allow us to add functional tests for EE only features in the
functional tests.

When pagination occurs python-gitlab uses the URL provided by the
GitLab server to use for the next request.

We had previously set the GitLab server configuraiton to say its URL
was `http://gitlab.test` which is not in DNS. Set the hostname
in the URL to `http://127.0.0.1:8080` which is the correct URL for the
GitLab server to be accessed while doing functional tests.

Closes: #1877

When running with docker-compose on Ubuntu 20.04 I got the error:

  $ docker-compose up
  ERROR: The Compose file './docker-compose.yml' is invalid because:
  networks.gitlab-network value Additional properties are not allowed ('name' was unexpected)

Changing the version in the docker-compose.yml file fro '3' to '3.5'
resolved the issue.