Expand conformance tests for multisampled renderbuffers

After #1935 is merged, tests need to be added for multisampled renderbuffer allocation. As @Richard-Yunchao pointed out in the working group, #1789 removed some tests of multisampled floating-point renderbuffers, which should be added back. (They can only be optional, since support for multisampled floating-point color renderbuffers is only made optional by the underlying extension.)

Review the changes planned in #1935 and the tests removed by #1789 before locating the conformance test entry points. Add coverage for multisampled renderbuffer allocation, including optional multisampled floating-point color renderbuffer cases, and verify the conformance tests pass with those cases handled as optional.

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