Test util create_room_as is_public option is misleading

[DMRobertson]

Defaults to True.

synapse/tests/rest/client/utils.py

Line 51 in 50022cf

is_public: bool = True,

Docstring says:

synapse/tests/rest/client/utils.py

Lines 65 to 67 in 50022cf

	is_public: If True, the `visibility` parameter will be set to the
	default (public). Otherwise, the `visibility` parameter will be set
	to "private".

But the only use is:

synapse/tests/rest/client/utils.py

Lines 80 to 81 in 50022cf

	if not is_public:
	content["visibility"] = "private"

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.