Test plan for Iterator.prototype.join

[bakkot]

The iterator join proposal is at stage 2.7 (not to be confused with the joint iteration proposal, newly at stage 3).

I came up with this list of tests by thinking about it, and then looking at the tests for Array.prototype.join.

• The usual boilerplate around name / length / enumerability / etc
• Expected results when joining iterator length 0, 1, 2, 3 with no provided separator
• As above but with a nonempty string as separator
• As above but with empty string as separator
• Throws if the receiver is not an object
• Coerces the separator argument to string, including null or undefined
• Coerces the iterator contents to string, except null and undefined
• null and undefined from the iterator produce empty strings in the result
• If ToString'ing the separator throws, the iterator gets closed
• If ToString'ing a value from the iterator throws, the iterator gets closed
• If the iterator throws, the iterator does not get closed
• If the iterator otherwise violates the iterator protocol, e.g. by returning null from its next, the iterator does not get closed
• If the iterator is exhausted, the iterator does not get closed
• .next is looked up only after ToString'ing the separator

[bakkot]

Tests: #4768