META: Unify workflow decisions between example repositories

[greg-1-anderson]

See #107 for background.

• Run or skip tests on non-pull-request builds: Logic for triggering Behat tests example-wordpress-composer#40
• Backup site before tests, restore after tests Decide policy on multidev mutability #113
• Run terminus wp or terminus drush prior to starting behat tests
• Delete and re-create test admin user prior to starting behat tests