Add rake task for running plugin tests.

This makes it possible to run Discourse tests with plugins loaded by setting `LOAD_PLUGINS=true`, as well as to run specs in the `spec/` directory of plugins by running either `rake plugin:spec` for all plugins or `rake plugin:spec[pluginname]` for a specific plugin.

Since most plugins aren't going to be complicated enough to require sub-directories inside of the `spec/` folder I don't recursively glob, all specs need to be directly in the `spec/` directory.

So it becomes easy to write tests for plugin functionality like this one:

```ruby
```