Make feature test app teardown more reliable, and tests faster

- don't spin up app and chromedriver between each test
    - catching signals also tears down the app
    - do layout reset between tests, but assume that tests will not leave a modal opened.

 Use selenium built-in waiting function and fix flakiness around clicking the alertify OK button

    - we think the OK button does not have its event bound when it is created.

If you see more flakiness around clicking the alertify OK button, let us know. The element is clickable but we have to arbitrarily wait for the event to be bound and that timing may vary system to system.

The feature tests are about 7 seconds faster now.

Tira & Joao

web/pgadmin/feature_tests/connect_to_server_feature_test.py

@@ -40,7 +40,6 @@ class ConnectsToServerFeatureTest(BaseFeatureTest):
 
     def tearDown(self):
         self.page.remove_server(self.server)
-        self.app_starter.stop_app()
 
         connection = test_utils.get_db_connection(self.server['db'],
                                                   self.server['username'],

web/pgadmin/feature_tests/template_selection_feature_test.py

@@ -1,4 +1,3 @@
-from selenium import webdriver
 from selenium.webdriver import ActionChains
 
 from regression import test_utils
@@ -44,7 +43,6 @@ class TemplateSelectionFeatureTest(BaseFeatureTest):
     def tearDown(self):
         self.page.find_by_xpath("//button[contains(.,'Cancel')]").click()
         self.page.remove_server(self.server)
-        self.app_starter.stop_app()
         connection = test_utils.get_db_connection(self.server['db'],
                                                   self.server['username'],
                                                   self.server['db_password'],