Test multiBinaryParts must be called twice with different arguments

[jhendrixMSFT]

For reference: https://github.com/Azure/cadl-ranch/blob/main/packages/cadl-ranch-specs/http/payload/multipart/main.tsp#L246-L250

The endpoint must be called twice with different arguments which is confusing and not documented on the operation. In a conversation with @timotheeguerin tests aren't supposed to follow this pattern.

[msyyc]

picture is optional:

cadl-ranch/packages/cadl-ranch-specs/http/payload/multipart/main.tsp

Lines 42 to 45 in 6a16166

	model MultiBinaryPartsRequest {
	profileImage: bytes;
	picture?: bytes;
	}

If you send twice that one has picture and the other has no picture, it definitely proves picture of your generated SDK is optional; if only send once with picture or with no picture, it is not enough to prove generated SDK is right.

[timotheeguerin]

Test should NOT be needing ever to be called twice this should be converted to 2 test
This means you are describing 2 feature in one and is terrible for new emitters to write test and understand that as well