[refactor] Added SeleniumTestMixin.get_webdriver method

Added "SeleniumTestMixin.get_webdriver" public method which
can be used by inherited class to provision extra web driver
instance.

(cherry picked from commit faa1616)

Author: pandafy

openwisp_utils/tests/selenium.py

@@ -81,10 +81,13 @@ def _setup_and_call(self, result, debug=False):
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
+        cls.web_driver = cls.get_webdriver()
+
+    @classmethod
+    def get_webdriver(cls):
         if cls.browser == "firefox":
-            cls.web_driver = cls.get_firefox_webdriver()
-        else:
-            cls.web_driver = cls.get_chrome_webdriver()
+            return cls.get_firefox_webdriver()
+        return cls.get_chrome_webdriver()
 
     @classmethod
     def get_firefox_webdriver(cls):