Count for multiple similar generated testcases

[RagnarGrootKoerkamp]

Instead of doing

- test: generate.py {seed:1}
- test: generate.py {seed:2}
- test: generate.py {seed:3}

it would be nicer to support something like

- test:
    input: generate.py {seed}
    count: 3

[mzuenni]

Related to #162 ?

How about something like this:

- test: generate.py {seed:[1,2,3]}  //1,2,3
- test: generate.py {seed:[1,..,3]} //1,2,3

You won't be able to generate multiple test cases with the same seed like this, but I don't think this is an issue. And it would allow you to generate multiple test cases with a selected set of seeds.

[RagnarGrootKoerkamp]

See #271