[css-grid] grid-float-001, grid-inline-float-001, grid-clear-001 and grid-inline-clear-001: review request

Hi Gérard,

following the line of the previous tests, I've 4 new tests to check that
"float" and "clear" properties don't apply to grid items.


* grid-float-001:
  * Source:
https://github.com/mrego/csswg-test/blob/css-grid-float-clear-tests/css-grid-1/grid-model/grid-float-001.xht
  * View:
https://rawgit.com/mrego/csswg-test/css-grid-float-clear-tests/css-grid-1/grid-model/grid-float-001.xht

* grid-inline-float-001:
  * Source:
https://github.com/mrego/csswg-test/blob/css-grid-float-clear-tests/css-grid-1/grid-model/grid-inline-float-001.xht
  * View:
https://rawgit.com/mrego/csswg-test/css-grid-float-clear-tests/css-grid-1/grid-model/grid-inline-float-001.xht

* grid-clear-001:
  * Source:
https://github.com/mrego/csswg-test/blob/css-grid-float-clear-tests/css-grid-1/grid-model/grid-clear-001.xht
  * View:
https://rawgit.com/mrego/csswg-test/css-grid-float-clear-tests/css-grid-1/grid-model/grid-clear-001.xht

* grid-inline-clear-001:
  * Source:
https://github.com/mrego/csswg-test/blob/css-grid-float-clear-tests/css-grid-1/grid-model/grid-inline-clear-001.xht
  * View:
https://rawgit.com/mrego/csswg-test/css-grid-float-clear-tests/css-grid-1/grid-model/grid-inline-clear-001.xht


Thank you very much,
  Rego