Add unit test to make sure all sections are present

Add a unit test that will check to make sure all top-level headings
and sub-headings are present.

The downside of this test is that it's going to fail on *every* merged
spec if we add or change a heading to the template.

Co-authored-by: Russell Bryant <rbryant@redhat.com>
Change-Id: I1c15282e166fb3fe2e60fcf063a6cd822a3411ff