Make be_an_existing_X matchers more useful

[mvz]

Summary

It should be possible to use matchers like be_an_existing_file everywhere in a project's spec suite, even outside an Aruba spec.

Expected Behavior

Just calling require "aruba/rspec" or similar should allow one to use be_an_existing_file in any spec.

Current Behavior

Calling this helper fails because it can't find stop_all_commands.

Possible Solution

• Don't stop commands
• Call file? helper in a way that doesn't require it to be a local method.

Context & Motivation

I'm porting a project from minitest to rspec + aruba, and wanted to use be_an_exsting_file to replace minitest's path_must_exist matcher.