[ECO-4757] Align ably-js tests with spec items

[VeskeR]

This PR marks all ably-js tests in correspondence with the spec items each test covers. This is achieved by adding docstrings for test case functions with the following tags:

• @spec - The test case directly tests all the functionality documented in the spec item.
• @specpartial - The test case partially tests the functionality documented in the spec item. This can be due to the spec item having conditional statements, the spec item being too overloaded and covering points that could be split into multiple spec items, or the spec item being documented for the parent class but the test case tests the functionality for one of its descendant classes.
• @nospec - No corresponding spec item was found for the test.
• @specskip - The test case is skipped during CI, so we should not take spec items related to this test case into account when deciding on metrics for spec coverage/feature tracking.

This information will be used in ECO-4852 to catalog the current state of specification coverage by ably-js tests and decide on future refactorings/improvements to the specification in ECO-12.

Additionally, docstrings may include references to other related spec items. These spec items are not tested directly by the test case but may provide additional context for the test.