To be honest, an option to "change the text size and contrast" is only a very small part of accessibility rules.
Also becuase 99% of browses have option to increase font size by two keys (ctrl + ), so extra button for this is almost not needed. Contrast (mostly Inverse colors) - indeed useful for some people, especially if they don't have a browser plugin (addon) that will do it for them.
Much more important is show the website content is presented in general. Becuase some people use screen voice readers software.
For Firefox/Chrome there is addon Wave - must have if we talk about WCAG (!)
But none of online tester would replace real human audit.