Make functions compatible with PHPUnit 8.0

The latest version (8.0) of PHPUnit requires a return type void on tear down and setup functions.

https://github.com/sebastianbergmann/phpunit/blob/master/src/Framework/TestCase.php#L401

Author: jverdeyen

src/PantherTestCaseTrait.php

@@ -67,7 +67,7 @@ trait PantherTestCaseTrait
         'external_base_uri' => null,
     ];
 
-    public static function tearDownAfterClass()
+    public static function tearDownAfterClass(): void
     {
         if (self::$stopServerOnTeardown) {
             static::stopWebServer();

tests/ServerExtensionTest.php

@@ -18,7 +18,7 @@
 
 class ServerExtensionTest extends TestCase
 {
-    public static function tearDownAfterClass()
+    public static function tearDownAfterClass(): void
     {
         PantherTestCase::$stopServerOnTeardown = true;
     }

tests/ServerListenerTest.php

@@ -21,7 +21,7 @@
 
 class ServerListenerTest extends TestCase
 {
-    public static function tearDownAfterClass()
+    public static function tearDownAfterClass(): void
     {
         PantherTestCase::$stopServerOnTeardown = true;
     }

tests/WebDriver/WebDriverMouseTest.php

@@ -20,7 +20,7 @@
  */
 class WebDriverMouseTest extends TestCase
 {
-    public static function setUpBeforeClass()
+    public static function setUpBeforeClass(): void
     {
         self::createPantherClient()->request('GET', self::$baseUri.'/mouse.html');
     }