Add dry-run mode for fetchers

## Overview

We should add a dry-run mode for fetchers that mocks out the Python `request` and instead returns a response containing content provided as a test file that contains production-like data typically returned by a call to an API. This will allow us to provide mocked responses and allow for Travis to validate that our fetchers and checks work as expected as part of any commit pushed or PR submitted to a compliance repo.

## Requirements

- Add a "dry-run" mode
- Thinking `--evidence dry-run`, this way it can be used on a fetch and a check execution and should behave like a `local` execution except that data will come from a sample file.
- dry-run mode should mock requests and replace the responses with content provided by files containing production-like data.
- Execution should be otherwise seamless

## Approach

TBD

## Security and Privacy

TBD

## Test Plan

TBD