Overridden test methods require void return type hints

PHPUnit 8 introduced void return types on the following methods:

• setUpBeforeClass()
• setUp()
• assertPreConditions()
• assertPostConditions()
• tearDown()
• tearDownAfterClass()
• onNotSuccessfulTest()

For example,

  protected function setUp() {

becomes

  protected function setUp(): void {

In order for tests to be compatible with both PHPUnit 6 and 7 (used in Drupal 8) and PHPUnit 8 (used in Drupal 9), a backward compatibility shim was added so these typehints are not yet required, but this compatibility shim will be removed in Drupal 10, or earlier if a newer version of PHPUnit means it is no longer viable to support.

If a test class extends another test that already type hints void, the code will need to update to require at least PHP 7.1, then just add (): void { return type hint.