Feature: Domain Mapping Unit Tests

[cameronterry]

Adds unit testing to the plugin and the first round of unit tests. At the moment the setup is primarily aimed at working with VVV, and can be run through composer using the following scripts:

# Ensure the dependencies are in place.
composer install

# Setup the unit testing environment
composer run test-setup

# Run the unit tests
composer run test

Also added a Github action which should fire on the develop branch as well as any release/* branch and any PRs pointed at them. Naturally, due to the issues discovered below, it will fail for the time being.

So far, the following issues have been discovered:

• Media domains not working with switch_to_blog() #91
• get_rest_url() incorrect returns the admin domain in switch_to_blog() #92
• Primary domain set / unset not updating domain cache #93

[cameronterry]

Noting the failed unit tests, however merging with the release branch to begin the fixes noted in the description.

• Media domains not working with switch_to_blog() #91
• get_rest_url() incorrect returns the admin domain in switch_to_blog() #92
• Primary domain set / unset not updating domain cache #93

Once these are fixed, the unit test action should begin passing.