skype: new 'add yes' testcase
skype: new 'add no' testcase
skype: new 'groupchat invited' testcase
skype_parse_chat: read MEMBERS, not ACTIVEMEMBERS

This makes /invite working again. No idea when this got broken, but I
promise the next commit will add a tescase for it.

Also, with this, other groupchat members will be already present in the
gropuchat when we chain, finally solving the annoying fake joins
presenting till now.
skype_chat_invite: test this
test skype_call_ask_yes()
test skype_call_ask_no()
test skype_call_strerror()
Merge from vmiklos.