Is the goal to test specific actions or to test the result of a user action?

It would be to test that the built-in behavior for these actions is correct. However, looks like no behavior is required by the spec, all I can find is this:

It is RECOMMENDED for user agents to implement a default handler for the play and pause media session actions if none was provided for the active media session.

If no action has any required side effect when action handlers aren't added, then the only thing that could be tested is that action handlers are invoked, and that would be very surface level and not really worth testing.

Compare to keyboard input, where testing the side effects of keyboard input on form elements is much more interesting than checking that the right keyboard events are fired.