Auto merge of #4586 - lukaslueg:issue4534, r=alexcrichton

Report hint if single failure with --no-fail-fast

There are two things going on in #4534:

* The doctest-kind masks the unittest-kind if doctests are executed during `--no-fail-fast`, no doctests fail and tests errors have already piled up. This is bug, fixed.
* Cargo would previously not report any unittest-hints if running with `--no-fail-fast`. This behavior has been changed to report a specific hint iif exactly one error has bubbled up during `--no-fail-fast`. The hint in #4534 is now `...pass '--test integ'`.