smooth.method="density" doesn't work in 'roc'

```
roc(aSAH$outcome, aSAH$ndka, smooth=TRUE, smooth.method="density")
Error in match.fun(paste("bw", bw, sep = "."))(roc$predictor) : 
  need at least 2 data points
```

This is because roc$predictor is not set at the time smooth.roc is called.

Make sure to re-enable tests by removing the skip_if call in test-roc.R once fixed.