add a common way to identify "core plugins" for testing

[rjbs]

Using Module::Pluggable was, as it usually is, at least partly a mistake.

If you write a command that gets tested on the CPAN, and someone else writes a plugin, and the plugin break things, all your tests break, because the plugin is loaded by default by Pluggable.

It should be possible to give a set of core plugins that are used in testing, at least.