Separate allenact/allenact_plugins tests.

## Problem

Currently our tests for `allenact` have dependencies on frameworks in `allenact_plugins`. Ideally these should be self contained.

## Desired solution

Create `test` directories in `allenact` and `allenact_plugins` and create new tests in `allenact` that are not framework dependent.

## Additional context

That this is a good idea was pointed out by @jordis-ai2 in #270 .