Testing(plugins): think about testing for our SSG plugins

See my comment [here](https://github.com/gatsbyjs/gatsby/pull/20589#issuecomment-574521700).

Adding my thoughts about the Gatsby plugin for now:

We need to make sure our Gatsby plugin is aligned with latest Gatsby core changes. One way to do it is to have a testing project that automatically updates to the latest Gatsby version and run some tests with the latest version of the plugin.

Start by reviewing the Gatsby plugin repository and the linked Gatsby core discussion to understand the compatibility concern. Done means a testing project automatically tracks the latest Gatsby version and runs tests against the latest plugin version.

Written by the indexing model from the issue text.