[Spec change] Clarify default context behavior

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#623 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, we need to add new CTS tests for this change:

• Create several queues using the constructors that do not take a context parameter.
• Get the context from each of these queues using queue::get_context.
• Make sure all of those context objects compare equal.
• Make sure the set of devices in that context object is the same as the set of devices in the queue's platform.