Test page features
When rewriting links for top-level frame, we need to use links of the
form: ./foo_files/some_file.png
When rewriting links for other frames (i.e. for frame getting saved
into ./foo_files/bar.htm), we need to use links of the form:
./some_other_file.png (i.e. without "foo_files").
Test content
Content verification marker:
frames-nested.htm: 4388232f-8d45-4d2e-9807-721b381be153
