Add tests for Document#fgColor with special body elements.

https://html.spec.whatwg.org/multipage/#dom-document-fgcolor states:

The attributes of the Document object listed in the first column of the following table must reflect the content attribute on the body element with the name given in the corresponding cell in the second column on the same row, if the body element is a body element (as opposed to a frameset element). When there is no body element or if it is a frameset element, the attributes must instead return the empty string on getting and do nothing on setting.

In particular, the following should be tested:

• getting with no body or frameset element
• setting with no body or frameset element
• getting with both only a frameset element
• setting with both only a frameset element
• getting with both a body and a frameset element
• setting with both a body and a frameset element

See some existing tests in html/obsolete/requirements-for-implementations/other-elements-attributes-and-apis/.