It's the one labeled "This tests if promise domain
membership infects subscriptions made in handlers."

The inner handler wasn't ever executing, due to
some kind of circularity I think. Not sure. Now it's
executing but not with the right promise domain
(meaning promise domain reentry isn't happening).

When a handler is subscribed to a promise (using then()
or similar) and a promise domain is active, then when
the handler is invoked, the promise domain should be
made active again. This allows the domain to "infect"
handlers that are registered later, but in the same
scope.

Previously, some tests were promise-chaining handlers containing
testthat assertions, and then calling wait_for_it(). This would
cause those assertions to be printed but not treated as test
failures, because wait_for_it() doesn't error on unhandled
promise errors. With this change, wait_for_it() takes a promise
and will raise an error if the promise rejects.

The bigger part of the commit is revisiting all the existing
uses of wait_for_it() and passing in promises if appropriate.