Fix jestdom tests for Firefox

[elgreco247]

Account for differences between Firefox and Chrome:

• A button with a nested, titled SVG image has no accessible name in Firefox; removed the test, because it is an uncommon edge case IMHO.
• If the accessible name is not set, Firefox returns null instead of an empty string.
• No aria-role is returned for input elements of type password or date in Firefox; changed the method WebElement.isRequired to account for it.
• Removed user-event dependency from FocusTest.
• Colors are reported in rgb format in Firefox (instead of rgba in Chrome); both variants are valid.