Perform the tests on the package that the `tests` attribute is a child
of, i.e. if `discourseAllPlugins.tests` is built, the tests will run
with the `discourseAllPlugins` package, not the `discourse` package as
previously.
Assert that the PostgreSQL version being deployed is the one used
upstream. Allow the user to override this assertion, since it's not
always possible or preferable to use the recommended one.