interest-is-interesting: link tasks to tests

[ErikSchierboom]

Links the tests for the interest-is-interesting exercise to individual tasks.
If a test is linked to a task, the online editor can show the student which tasks has tests failing.

The tasks for this exercise are found in its instruction.md document.
A task's ID is the number they're prefixed with (so ## 2. Task title has ID 2).

To link a test in the exercise's tests to a task, you add a Task attribute to it, which takes a single int argument representing the task identifier.

For an example what that would look like, see how these tests are linked to these tasks.