Pseudocode for a PHPUnit test in an extension. Noe that CiviTester and CRM_Core_Transaction are not tied to PHPUnit in any way.

EndToEndExampleTest.php

<?php
class EndToEndExampleTeset extends PHPUnit_Framework_TestCase {
  
  public function setUp() {
    // Ensure that we are running an end-to-end test on a real site
    // where the extension has been enabled. If the expectations
    // aren't met, generate an error.
    CiviTester::e2e()->extension(‘org.civicrm.foo’)->apply();
  }
  
  public function testFoo() {
    // Send requests to the website.
    // Use $_CV['ADMIN_USER'], $_CV['ADMIN_PASS'], $_CV['DEMO_USER'], etc
  }

}

HeadlessExampleTest.php

<?php
class HeadlessExampleTest extends PHPUnit_Framework_TestCase {
  
  public function setUp() {
    // Ensure the headless DB has the Civi schema along with the extension.
    // If necessary, wipe/reset headless DB.
    CiviTester::headless()->extension(‘org.civicrm.foo’)->apply();

    // Wrap the test in a transaction so that we don't have to do any data cleanup.
    $this->tx = new CRM_Core_Transaction();
  }
  
  public function tearDown() {
    $this->tx->rollback();
    unset($this->tx);
  }
  
  public function testFoo() {
    // do stuff with the extension
  }

}