CI: Use `HOSTSUFFIX` instead of `TEST_NAMESPACE` for consistency

The following discussion from !623 (merged) should be addressed:

• @dmakovey started a discussion: (+3 comments)

  to get issues resolved with DNS - you'd have to trim HOSTSUFFIX mainly. TESTS_NAMESPACE is being used as default only when HOSTSUFFIX is not specified (i.e. scripts/test.sh is ran manually).

we've got TEST_NAMESPACE used in https://gitlab.com/gitlab-org/cloud-native/gitlab-operator/-/blob/master/.gitlab-ci.yml#L404-L412 whereas we should be using HOSTSUFFIX for consistency.