test() should error on uncaught output

```r
test(1, { cat("123\n"); TRUE })
# 123
```

That should error IMO, much as we error for uncaught warnings/messages:

```r
test(1, { message("123\n"); TRUE })
Test 1 produced 1 messages but expected 0
Expected: 
Observed: 123
```

This will help keep the test log clean and is also a best practice IMO.