How to reset the headless test db?
ah-ha!

In your test class edit the setUpHeadless() method which might look at first like this:

  public function setUpHeadless() {                                                                                                                                                                                                           
    // Civi\Test has many helpers, like install(), uninstall(), sql(), and sqlFile().
    // See: https://github.com/civicrm/org.civicrm.testapalooza/blob/master/civi-test.md
    return \Civi\Test::headless()
      -&gt;installMe(__DIR__)
      -&gt;apply();


But pass TRUE to apply(), ie

  public function setUpHeadless() {                                                                                                                                                                                                           
    // Civi\Test has many helpers, like install(), uninstall(), sql(), and sqlFile().
    // See: https://github.com/civicrm/org.civicrm.testapalooza/blob/master/civi-test.md
    return \Civi\Test::headless()
      -&gt;installMe(__DIR__)
      -&gt;apply(TRUE);


Ta-daa! And now your tests will run super slow as the schema will be reinstalled every time. But helps you out of  a spot, and you can of course remove that param.