page-experience: check for invalid font preloads

Is there a separate check for invalid preloads somewhere? Perhaps a preload of a value used in a font-face declaration but not preloaded as a font?

_Originally posted by @kristoferbaxter in https://github.com/ampproject/amp-toolbox/pull/1156#r589534227_

Start by locating the existing page-experience checks for preloads and compare their handling of values used in font-face declarations. Confirm whether invalid font preloads are already detected; if not, define the expected validation cases and the behavior that would mark the work complete.

Written by the indexing model from the issue text.