Migrate Element#to_subtype examples from Watir-Webdriver

The Watir-Webdriver spec has additional examples for how the Element#to_subtype method should handle input elements. I would like to migrate these examples to Watirspec as they identify some issues with the Watir-Classic implementation. In particular, the handling of button, text and file types fail in Watir-Classic.

Changes made from the Watir-Webdriver spec:
* The name of the file type example was corrected from "returns a TextField instance" to "returns a FileField instance"
* Included the reset and image types in the button instance example