title: Testing levels/groups in PHPUnit
body: <p>We already have some PHPUnit tests implemented on our codebase.  However, moving forward, in the interest of minimizing the time required to run tests, I'd like to separate out our tests into two groups, or levels:</p>
  
  <ol>
  <li>Tests that are very low-level and don't hit the test database (unit tests)</li>
  <li>Tests that are higher level and interact with the test database (sometimes called component or integration tests)</li>
  </ol>
  
  <p>This would allow us to just run the level 1 test when working on basic functionality, while running both level 1 and 2 when working on higher-level functionality, or before committing to the main line/build.</p>
  
  <p>How would I go about accomplishing this with PHPUnit?</p>