Execs.exec(ScenarioBuilder) chops off the first element of scenario.

When using `Execs.exec` on a Scenario builder, the first action in the chain is removed. It's unclear what the original reason for this was, but it doesn't seem like the right behaviour now (the first element in the chain in many of my scenarios is a `GroupStart` - so I end up with a `GroupEnd` but no `GroupStart`!). Removed this behaviour, as well as a redundant concatenation with `actionBuilders` (`exec` delegates to `chain`, which also does this)

Also added a unit test for this bug.