Add failure only reporter

[natebosch]

Closes #829

Copies the expanded reporter and test and removes output unrelated to
failed tests.

[natebosch]

Some details worth getting agreement on:

• Naming. I'm going with -r failures-only to be specific, but if anyone thinks we should do --quiet or anything else we should pick a good name to start with.
• This currently doesn't print skips since a skip doesn't cause a failure. We still show the number skipped at the end of the test.
• We do print the final line with the number of passing/failing/skipped tests. So there is some output even for fully passing suites. We could instead be totally silent for passing runs, but I think the main goal is to reduce noise, and the summary line carries a lot of signal.

[jakemac53]

Naming. I'm going with -r failures-only to be specific, but if anyone thinks we should do --quiet or anything else we should pick a good name to start with.

I could see doing --quiet but I think this is more consistent with how we generally configure reporters. So I think it is good as is.

This currently doesn't print skips since a skip doesn't cause a failure. We still show the number skipped at the end of the test.

SGTM

We do print the final line with the number of passing/failing/skipped tests. So there is some output even for fully passing suites. We could instead be totally silent for passing runs, but I think the main goal is to reduce noise, and the summary line carries a lot of signal.

I think its still worth printing the summary also, so SGTM

[natebosch]

We also print the time since starting the test run today - looking at this again I'm leaning towards removing that. In the compact reporter it makes sense because we keep updating the same display of the time on the current line in the typical case. In the expanded reporter it can be used to see the delta between a test start and test failure. Absent other information, knowing the time since test start for an arbitrary failure is not useful.

Knowing the full runtime could be useful, but can also be easily measured externally (as long as it's not a problem picking up the test runner overhead).