Simplification of css_defaultDisplay helper

* Fix #12723: use getDefaultComputedStyle method in FF to circumvent iframe path
* Rename helper "css_defaultDisplay" to "defaultDisplay" since it used only in css module
* Take away fix for #10227 ticket, as it is no longer needed.
* Improve test for #10227 fix - iframe of defaultDisplay helper appended to html element,
  so check correct default display for html-element too
* Simplify iframe creation