Add a dedicated @PluginTestFruit annotation class

Follow-up from 🌱 [meta] Introduce specific annotations for each plugin type Fixed .

Talking with tim.plunkett on IRC, he suggested that if this happens at all, it should wait until uses of @Plugin are otherwise eliminated throughout core:

8:54 Les_Lim timplunkett: for https://drupal.org/node/1966246, should all the plugin_test plugins have custom annotation classes?
8:54 Druplicon https://drupal.org/node/1966246 => [meta] Introduce specific annotations for each plugin type 🌱 [meta] Introduce specific annotations for each plugin type Fixed => 50 comments, 10 IRC mentions
8:55 timplunkett Les_Lim: well if we want to kill @Plugin completely, yes. but idk if that will happen...
8:55 Les_Lim timplunkett: Plugin/plugin_test/fruit uses @Plugin, but there's also "Plugin/plugin_test/custom_annotation" that tests @PluginExample
8:56 timplunkett Les_Lim: yeah @PluginExample was the original test coverage for using something custom
8:56 Les_Lim timplunkett: so it seems like just testing @Plugin is intentional
8:56 timplunkett Les_Lim: well, until we get rid of it, we should test it. so i guess those would be the last ones

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