Add specs for existing `Fog::Bin` API

This adds tests to check the current implementation of
the `Fog::Bin` class which is used as the superclass for
all the provider specific classes used by the `fog` binary.

The implementation is due to be refactored to solve
fog/fog-core#105 but this adds something to assist there.

**Important** this is based on `v1.25.0` since the bin
code was moved to `fog-core` following that. That
change is due to be reverted.