add alert test case (#579)

* add alert test case

* add comment

Author: KazuCocoa

ios_tests/lib/ios/specs/ios/xcuitest_gestures.rb

@@ -89,6 +89,8 @@ def open_alert_custom
     alert action: 'dismiss'
 
     open_alert_custom
+    list = alert action: 'getButtons'
+    list.must_equal %w(Button1 Button2)
     alert action: 'accept', button_label: 'Button1'
   end
 

lib/appium_lib/ios/xcuitest_gestures.rb

@@ -146,7 +146,8 @@ def select_picker_wheel(element:, order:, offset: nil)
       # @param action [String] The following actions are supported: accept, dismiss and getButtons. Mandatory parameter
       # @param button_label [String] The label text of an existing alert button to click on.
       #   This is an optional parameter and is only valid in combination with accept and dismiss actions.
-      # @return nil or Selenium::WebDriver::Error::NoSuchAlertError if no action sheet or alert
+      # @return {} or Selenium::WebDriver::Error::NoSuchAlertError if no action sheet or alert
+      #   or button labels if action is equal to getButtons.
       #
       #   ```ruby
       #   alert action: "accept"