[css-grid] Fix positioned children in RTL direction
We were not honouring RTL for positioned children, we didn't have
any specific code for that so a lot of cases were not working properly.
Refactored the code in offsetAndBreadthForPositionedChild() in order to
calculate offset and breadth in a more clean way that works for both
LTR and RTL.
Added RTL cases for most of the positioned tests, which are now passing
with this patch.
Converted some of the tests to testharness tests to avoid the need of
updating the expected results.
BUG=
568882