Vulkan multiimport diffctx fix

[dcrawleyqti]

The multi import diff ctx portion of the Vulkan test mistakenly instantiates clExternalImportableSemaphore objects as wrappers for semaphores that will be exported (vkCl2VkSemaphore). This change corrects this and mirrors the correct behavior found in the multi import same ctx test.

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[kpet]

May be superseded by https://github.com/KhronosGroup/OpenCL-CTS/pull/2243

[dcrawleyqti]

superseded by #2300