Actions() schema getter added to Charm interface; requested changes made

Changes:
 - charm/actions.go stripBadInterfaces now simplified into a "cleanse" function.
 - Better tests added for cleanse.
 - A few excess tests removed from charm_test.go.
It does, but make is actually quite clear here. (I generally favour the ...{} form, but am nor sure if we have a common style on this).