Assert that the login succeeded for more helpful error messages

Change-Id: I9f985d31de86fe2a062d10ca3f51fb2d7c727798

tests/browser/features/step_definitions/common_steps.rb

@@ -7,6 +7,9 @@ end
 
 Given(/^I am logged in$/) do
 	visit(LoginPage).login_with(@mediawiki_username, @mediawiki_password)
+	# Assert that login worked
+	loggedin = !@browser.execute_script( "return mw.user.isAnon();" )
+	loggedin.should be_true
 end
 
 def language_to_code(language)