Add support for env and fatalError options

[romaia]

env allows to specify environment variables that will be passed to the command being executed.

fatalError allows to change the default behaviour that will abort the test execution, allowing for expected failures to not break k6.

When the command exits with a non zero error code, the stderr will be returned instead of the stdout

[CLAassistant]

All committers have signed the CLA.

[frankisawesome]

The env option here would be quite awesome, wondering if there's a chance to decouple the two features of this PR and get env options in sooner @romaia ?

[pablochacin]

Hi @romaia. We are closing this PR due to inactivity. If you are still interested in moving this PR forward based on our discussion please re-open it.