Protection against premature page operations after form submission

It appears, that some drivers (e.g. MinkSeleniumDriver) are performing form submission slower (a bit) when dedicated `submit` method is used, rather when a `click` is made on `<input type='submit' .../>` button. For that reason there might be a delay before actual page is available to the driver after form submission was made.

In reality a `testFormSubmit` test was failing because of that. Just to be on the safe side I've added protection (using `wait` method) to both `testFormSubmit` and `testBasicForm` tests.