[Spec change] Add KHR for default context

[sycl-issue-bot[bot]]

Please review whether https://github.com/KhronosGroup/SYCL-Docs/pull/624 by @gmlueck requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue. If no changes are required: Close this issue and proceed with the spec PR.

[gmlueck]

Yes, this PR requires additional CTS coverage:

• Call platform::khr_get_default_context for the tested platform.
• Create a queue using each constructor that that does not take an explicit context parameter.
• Call queue::get_context on each of those queues and make sure the object compares equal to the object returned from platform::khr_get_default_context.

Note that this is a KHR, so the testing needs to be conditionally enabled based on whether the implementation defines the SYCL_KHR_DEFAULT_CONTEXT macro.