Tests that source data are extracted correctly from external stylesheets in UTF-8 with BOM. Bug 59322.

[expanded] 
element.style { ()

[expanded] 
h1 { (parse-utf8-…-main.css:3 -> parse-utf8-bom-main.css:3:5)
    color: red;

[expanded] 
h1 { (user agent stylesheet)
    display: block;
    font-size: 2em;
    margin-block-start: 0.67em;
    margin-block-end: 0.67em;
    margin-inline-start: 0px;
    margin-inline-end: 0px;
    font-weight: bold;
    unicode-bidi: isolate;