test environment variable with readme updates

[cfarm]

instead of typing workon oah-tests everytime you need to run browser tests, this automagically does it for you when you cd to the test folder. 🎉

Additions

• add environment variable for the test folder for easy browser testing locally

Changes

• add instructions for copying .env_SAMPLE plus table of contents for readme

Testing

• checkout branch
• follow instructions for copying the .env_SAMPLE file in the readme here
• cd to test directory to confirm oah-tests virtual env will run

Review

• @virginiacc

Screenshots

Checklist

• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows the standards laid out in the front end playbook
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)

[anselmbradford]

Can the values in test/.env_SAMPLE just be added to https://github.com/cfpb/owning-a-home/blob/master/.env_SAMPLE so there's only one .env_SAMPLE in the project?