It's very easy to forget to `await` or `return` the promise returned
from `expectAsync`. When that happens, the expectation failure will
occur after the spec or suite's result has been reported to reporters,
and the failure will typically not be shown to the user. This change
adds a top-level suite failure in that case, similar to the way we
report unhandled exceptions or promise rejections that occur after the
runable completes. Adding the error at the top level gives us the best
chance of getting in before the set of failures we add it to is sent
to reporters.
See #1752.
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