Test: assert namespaces populated before publishing a DB

[spiffcs]

What would you like to be added:
An integration test or self validation should be added to grype-db that checks if the database being published includes all known namespaces (or has an extra one).

Why is this needed:
Previously grype-db have been published that were missing namespaces or had 0 entries in a namespace. This check will ensure that we publish complete DB with respect to known namespaces.

Additional context:
Tools team can provide more context via the Debian Post-Mortem document

[wagoodman]

I've got part of a solution in with #192 but am putting this back in the backlog until the namespaces can be sourced with a single source of truth (described in the PR description).