[BUGFIX] Respect overridden properties in AbstractPageTree

This changes all property accesses within AbstractPageTree to
use static instead of self. Since the class is abstract and
therefore subject to class inheritance, its properties may also
change in subclasses. Using self to access them would hide
potentially overridden properties (late static binding).

Classes/Core/Acceptance/Helper/AbstractPageTree.php

@@ -52,7 +52,7 @@ public function openPath(array $path)
         foreach ($path as $pageName) {
             $context = $this->ensureTreeNodeIsOpen($pageName, $context);
         }
-        $context->findElement(\Facebook\WebDriver\WebDriverBy::cssSelector(self::$treeItemAnchorSelector))->click();
+        $context->findElement(\Facebook\WebDriver\WebDriverBy::cssSelector(static::$treeItemAnchorSelector))->click();
     }
 
     /**
@@ -65,7 +65,7 @@ public function getPageTreeElement()
         $I = $this->tester;
         $I->switchToIFrame();
         return $I->executeInSelenium(function (\Facebook\WebDriver\Remote\RemoteWebDriver $webdriver) {
-            return $webdriver->findElement(\Facebook\WebDriver\WebDriverBy::cssSelector(self::$pageTreeSelector));
+            return $webdriver->findElement(\Facebook\WebDriver\WebDriverBy::cssSelector(static::$pageTreeSelector));
         });
     }
 
@@ -79,7 +79,7 @@ public function getPageTreeElement()
     protected function ensureTreeNodeIsOpen(string $nodeText, RemoteWebElement $context)
     {
         $I = $this->tester;
-        $I->see($nodeText, self::$treeItemSelector);
+        $I->see($nodeText, static::$treeItemSelector);
 
         /** @var RemoteWebElement $context */
         $context = $I->executeInSelenium(function () use (