Test util create_room_as is_public option is misleading #10951

Defaults to True.

Docstring says:

But the only use is:

Moreover, the CS spec says of visibility that

Rooms default to private visibility if this key is not included.

Suggest:

• Change is_public to Optional[bool], default None
• if is_public is False, set visibility to private
• elif is_public is True, set visibility to public
• else do nothing.

Could alternatively leave it as a bool and fix it to actually set visibility to public when True. That would probably require fixing a bunch of tests to explicitly set is_public: False.

Spotted by @AndrewFerr in #10930.