Workflow alias

Similarly to #96, I would like to test a (sub-)workflow under an alias. That is because I run the sub-workflow in two different fashions in my pipeline and I have my configuration set up accordingly.

I'm not sure about the best syntax, but it could either be:

```groovy
workflow "SUBFLOW as SUBFLOW1"
```

or

```groovy
when {
    workflow {
        alias "SUBFLOW1"
```

or something like that.