`/css/css-fonts/generic-family-keywords-003.html` is intermittent

[mrobinson]

This test seems to assume that web fonts are going to be immediately available as soon as their stylesheet is added to the page. This isn't how Servo works, even when the data for the URL is already loaded.

Example failure:

OK /css/css-fonts/generic-family-keywords-003.html

    FAIL [expected PASS] subtest: @font-face matching for quoted and unquoted serif (drawing text in a canvas)

    assert_equals: quoted serif matches  @font-face rule expected 125 but got 40