Test that display_order is not None in comps.xml

[pulpbot]

Author: @fao89 (fao89)

Redmine Issue: 6340, https://pulp.plan.io/issues/6340

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1. Create a rpm repo and a remote.
2. Sync the repo with the remote.
3. Publish and distribute the repo.
4. Check whether display_order is not None in comps.xml.

Reference: #1787

similar test: ValidateNoChecksumTagTestCase - https://github.com/pulp/pulp_rpm/blob/master/pulp_rpm/tests/functional/api/test_publish.py#L232

Fixture: https://repos.fedorapeople.org/pulp/pulp/fixtures/srpm/