Automate test(s) for scripted fields

[LeeDr]

We have broken scripted fields on a couple of occasions and could miss it in manual testing so we should automate a couple of tests.

1. We need to make sure a scripted field shows up and has the correct values in Discover
2. and can be used in a Metric Aggregation in a Visualization and shows the correct values
3. should test both 'expression' and 'painless' script types

I think we already have a Discover test that verifies a document's contents (to see that it sorted by date I think). So a copy of that test after a scripted field has been created should verify the value of the scripted field.