Clean up output matchers

## Summary

There are several matchers defined by Aruba that are very similar to matchers provided by RSpec. For example:

* `include_output_string` and its aliases `an_output_string_including` and `file_content_including`. These seem similar to the `include` or perhaps `match` matchers.
* `include_an_object`. This seems similar to `include`.

## Expected Behavior

Aruba should use default RSpec matchers where possible to make its internal code comprehensible and to provide good examples for its users. Also, Aruba should not define generic matchers that may have to be supported beyond their internal use.

## Current Behavior

There's a proliferation of matchers that increase the maintenance burden.

## Possible Solution

Remove matchers that don't pull their weight.

## Context & Motivation

Improving the output of failing steps is hard because there are many custom matchers that all need to be checked.

No files or tests are named; start by locating the definitions and usages of include_output_string, an_output_string_including, file_content_including, and include_an_object. Compare those usages with RSpec's default matchers, and consider the work done when redundant custom matchers are removed without losing relevant matcher behavior.

Written by the indexing model from the issue text.