Report all assertion failures

With #259, we have moved from only printing the last assertion failure, to only printing the first.

Once #243 (chainable methods) happens, I think we should allow printing of every failed assertion to give the most information possible.

``` js
test.verbose('title', t => {
t.fail('a');
t.fail('b');
});
```

Perhaps `only` mode should also just imply `verbose` mode (you should have way less noise with a single test).

Start by reading the changes described in #259 and the proposed chainable methods in #243, then reproduce the test.verbose example from the issue. Determine the expected behavior for reporting every failed assertion and whether only mode should imply verbose mode; done means the behavior is specified and covered for both modes.

Written by the indexing model from the issue text.