feat: Improve plugin dependency requirements

[simondotm]

We're running unnecessary init generator calls in function & sync generators that ensure packages exist in the workspace.
This has a side effect of adding dependencies that users may not want in their workspace.

This PR:

• Removes calls to init generator for functions + sync (on the assumption that app generator has to have previously run, and deps were added at that point)
• Removes plugin addition of firebase-functions-test dependency. The plugin doesn't require it, and it's optional for firebase development, so this way users are free to add it themselves if/when they need to

Fixes #269