[css-grid] display-grid,

Hi Gérard,

here are my first 3 tests checking the new values "grid" and
"inline-grid" for "display" property.

I've joint them in a single pull request but I can split it in 3
different pull-requests as you wish.

Pull request: https://github.com/w3c/csswg-test/pull/626

Tests sources:
*
https://github.com/Igalia/csswg-test/blob/css-grid-display-tests/css-grid-1/grid-model/display-grid.xht
*
https://github.com/Igalia/csswg-test/blob/css-grid-display-tests/css-grid-1/grid-model/display-inline-grid.xht
*
https://github.com/Igalia/csswg-test/blob/css-grid-display-tests/css-grid-1/grid-model/parsing-display-grid-inline.xht

Thanks,
   Rego