[discuss] Improve testing support for content packages

For content packages, at the moment we only validate their format and test that they can be installed.

It would be nice to allow some additional tests to validate their contents, for example with tests for dashboards (https://github.com/elastic/elastic-package/issues/1705).

Start by reviewing how content packages are currently validated for format and installation. Use the dashboard testing discussion in issue #1705 as a concrete example, then define which additional content checks should be supported and how their successful completion will be verified.

Written by the indexing model from the issue text.