DevTools: shard inspector/elements tests for faster execution.

LayoutTests/inspector/elements/styles-2/perform-undo-perform-of-mergable-action-expected.txt

 Tests that perform-undo-perform of the mergeable action does not crash.
 
 Initial value
 [expanded] 
 element.style  { ()
 
 [expanded] 
 .container  { (perform-undo-pe…-action.html:7 -> perform-undo-perform-of-mergable-action.html:7:1)
     font-weight: bold;
 
@@ skipping 31 matching lines @@
 
 [expanded] 
 .container  { (perform-undo-pe…-action.html:7 -> perform-undo-perform-of-mergable-action.html:7:1)
     font-weight: normal;
 
 [expanded] 
 div  { (user agent stylesheet)
     display: block;