Added two more tests for multiple and no api-endpoints

Added two more api-endpoint test cases for multiple endpoints (and using only the first one) and no endpoints (so automatically refresh and return the first endpoint then).
I find such function confusing: adds more endpoint addresses or removes all in case of nil.
The two separate function would be more appropriate here.
But even if we stay the above function, I would do nothing if called modifyAddresses(c, []string{}),
and delete all addresses only if called modifyAddresses(c, nil).

Anyway, the current comment is invalid, because now all addresses are removed in case of []string{}, too.