Install geth if `geth --dev` is used for testing

[fubuloubu]

Using the geth plugin for testing won't function without geth binary present, so make sure we add this (and cache it) if we detect that plugin in use

Not as sure how to do this one because ape-geth is installed by default, a little deeper introspection needed to show that we are indeed using geth --dev within the test suite

[antazoey]

we could be clever, but it might be better to just have a use_geth: bool input.
it is only a small gotcha and keeps it simpler and more accurate

[antazoey]

alternatively, to be more like the other provider issues here, we could force the user to put geth in the ape-plugins-listto get geth installed correctly.

[antazoey]

we may want to close all of these as "wont do"