Speed up tests

## Summary

Aruba's tests take too long. Especially on JRuby, but on MRI they're still take quite long. Let's make them faster.

## Possible Solutions

* Test edge cases as unit tests instead of using an end-to-end test. This will probably mean moving them from cucumber to rspec.
* Figure out a way to reduce indirection. Current feature tests often do a nested cucumber run, which itself does a nested run of some Ruby program.

## Steps to Reproduce (for bugs)

Look at [a typical Travis build](https://travis-ci.org/cucumber/aruba/builds/631370114).

## Context & Motivation

Development is slow when you need to wait that long for a build to finish.

Start with the linked typical Travis build and identify which Cucumber feature tests and nested Ruby-program runs dominate time on JRuby and MRI. Compare those paths with the existing RSpec tests; done means reducing build time by replacing suitable edge-case end-to-end coverage with unit tests or removing unnecessary nested runs.

Written by the indexing model from the issue text.