The previous assert step was optimistically assuming that the async
continuation that writes the value of
`FakeImportingPlayer.ImportedScore` always completes before it, but
that's not necessarily true even if the continuation is instant (it is
still subject to things like task scheduling and TPL thread pool
limits).
To ensure no spurious failures, swap out the assert step for an until
step instead.
Was previously not handled at all, therefore displaying request failures
in the test log output. While that was mostly a red herring and
shouldn't have caused any actual *test* failures, it is still better to
handle this explicitly in a realistic manner.