Tests that rules from imported stylesheets are correctly shown and are editable in inspector.

Rules before toggling:
[expanded] 
element.style { ()

[expanded] 
.square { (imported-st…sheet.css:2 -> imported-stylesheet.css:2:2)
    background-color: red;
    display: inline-block;

[expanded] 
div { (user agent stylesheet)
/-- overloaded --/     display: block;

Rules after toggling:
[expanded] 
element.style { ()

[expanded] 
.square { (imported-st…sheet.css:2 -> imported-stylesheet.css:2:2)
/-- overloaded --/ /-- disabled --/     /* background-color: red; */
    display: inline-block;

[expanded] 
div { (user agent stylesheet)
/-- overloaded --/     display: block;