browser test: fix logic to find edit action

The simple math test failed to find the edit link on a non existent
page. It was looking for the text 'Create source' from Vector which is
now simply 'Create'.

Change the PageObject selector to use a CSS selector, more robust to
text changes.

Change-Id: Ifcbd07da8e343b5697fcff6af6ffac90ff4d6420

tests/browser/features/math.feature

@@ -15,7 +15,7 @@ Feature: Math
   Scenario: Display simple math
     Given I am logged in
       And I am at page that does not exist
-    When I click link Create source
+    When I click link Create
       And I type <math>3 + 2</math>
       And I click Preview
     Then the page should contain an img tag

tests/browser/features/step_definitions/math_steps.rb

@@ -14,8 +14,8 @@ Given(/^I am at page that does not exist$/) do
   visit(DoesNotExistPage, using_params: { page_name: @random_string })
 end
 
-When(/^I click link Create source$/) do
-  on(DoesNotExistPage).create_source_element.when_present.click
+When(/^I click link Create$/) do
+  on(DoesNotExistPage).action_edit_element.when_present.click
 end
 
 When(/^I click Preview$/) do

tests/browser/features/support/pages/does_not_exist_page.rb

@@ -14,5 +14,5 @@ class DoesNotExistPage
 
   page_url '<%=params[:page_name]%>'
 
-  a(:create_source, text: 'Create source')
+  a(:action_edit, css: 'li#ca-edit > span > a')
 end