Example cicle.yml file for unit testing a Wordpress plugin with wp-cli

circle.yml

## Customize the test machine
machine:
  timezone:
      America/New_York # Set the timezone

    # Version of ruby to use
  php:
    version:
      5.6.5

  environment:
    WP_VERSION: latest
    CIRCLE_ENV: test
    WP_MULTISITE: 0 
    WP_CORE_DIR: /home/ubuntu/wordpress-develop 
    WP_TESTS_DIR: /home/ubuntu/wordpress-develop/tests/phpunit 
    plugin_loc: /home/ubuntu/$CIRCLE_PROJECT_REPONAME 
    plugin_slug: $CIRCLE_PROJECT_REPONAME 
    plugin_dir: /home/ubuntu/wordpress-develop/src/wp-content/plugins/$plugin_slug 
    plugin_tests_dir: /home/ubuntu/wordpress-develop/src/wp-content/plugins/$plugin_slug/tests



## Customize dependencies
dependencies:
  pre:
    #enable xdebug.  LINE 1/2 to uncomment if you want to run a code coverage report.
    # - sed -i 's/^;//' ~/.phpenv/versions/$(phpenv global)/etc/conf.d/xdebug.ini
    #setup WP install
    - git clone git://develop.git.wordpress.org/ $WP_CORE_DIR;
    - cd $WP_CORE_DIR && cp wp-tests-config-sample.php wp-tests-config.php && sed -i "s/youremptytestdbnamehere/wordpress_test/" wp-tests-config.php && sed -i "s/yourusernamehere/root/" wp-tests-config.php && sed -i "s/yourpasswordhere//" wp-tests-config.php;
    # move plugin into tests/src
    - mv $plugin_loc $plugin_dir;
    # set up database
    - mysql -e 'CREATE DATABASE wordpress_test;' -uroot;
    # setup phpunit
    - wget https://phar.phpunit.de/phpunit.phar && chmod +x phpunit.phar && mv phpunit.phar /home/ubuntu/.phpenv/shims/phpunit

  override:
    # Circleci build fails if no composer.json found
    - touch composer.json

## tests override
test:
  override:
    # comment out the below line to run a code coverage report.
    - cd $plugin_tests_dir; cd ..; pwd; composer install; ls; phpunit
    ## LINE 2/2 to uncomment if you want to run a code coverage report.
    # - cd $plugin_tests_dir; phpunit --coverage-html $CIRCLE_ARTIFACTS

Adapted from this. I hacked the last line a bit to match my directory structure.