Add acceptance test for method outcomes

This was written as part of updating the documentation to reflect the current
state.

Ref: https://dev.cfengine.com/issues/6538
Why is this linux-specific ?  Surely that makes the test broken on non-linux !
The command "false" should work on all platforms (if only, with useshell, as a shell builtin); and it probably is /bin/false on every Unix.