META: Unify workflow decisions between example repositories

[greg-1-anderson]

See https://github.com/pantheon-systems/example-drops-8-composer/pull/107 for background.

• [ ] Run or skip tests on non-pull-request builds: https://github.com/pantheon-systems/example-wordpress-composer/issues/40
• [ ] Backup site before tests, restore after tests https://github.com/pantheon-systems/example-drops-8-composer/issues/113
• [ ] Run terminus wp or terminus drush prior to starting behat tests
• [ ] Delete and re-create test admin user prior to starting behat tests