Missing negative case for clCreateContext function

Attempt to create context with multiple devices associated with different platforms should have separate negative case, something like:

`CL_INVALID_DEVICE if the platform associated with devices in devices list is not the same.`

Additional remarks:

1) Creating multi-device-platform context is impossible. I think spec could do more about explaining such a case. Description from the devices argument could be extended, currently it says:

`devices is a pointer to a list of unique devices returned by clGetDeviceIDs or sub-devices created by clCreateSubDevices for a platform.`

2)  At my machine attempt to create context with two devices from nvidia and intel platform crashes. I could only presume this is not an isolated case. Here is my branch to test such case with `test_context`:

https://github.com/shajder/OpenCL-CTS/tree/context_negative_create

Start with the clCreateContext specification and the devices argument description, then review the linked test_context branch for the proposed negative case. Done means the specification separately states the cross-platform-device error and clarifies that devices must belong to one platform; verify the wording against the existing API tests.

Written by the indexing model from the issue text.