Fix payload value query for external semaphores

[ahesham-arm]

When creating a CL semaphore object from a Vulkan semaphore one, we explicitly pass -1 as the file descriptor value in the case of VULKAN_EXTERNAL_SEMAPHORE_HANDLE_TYPE_SYNC_FD. According to the CL specification:

The special value -1 for fd is treated like a valid sync file
descriptor referring to an object that has already signaled. The
import operation will succeed and the semaphore will have a
temporarily imported payload as if a valid file descriptor had
been provided.

The test currently checks that the semaphore payload is unsignalled, unconditionally, which is incorrect.

Changed the test to check for the correct expected payload value.