[css-grid] Migrate grid-content-alignment-and-self-alignment-spanning…

….html to WPT

Migrate this test out of blink/web_tests/fast/css-grid-layout
and into the WPT-specific directory, adding links to the relevant
specs and a test assertion describing its purpose.

Bug: 767015, 1063749
Change-Id: I772e4c3651a35e0d47e945418aeb408250e50fa1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2152450
Reviewed-by: Manuel Rego <rego@igalia.com>
Reviewed-by: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Cr-Commit-Position: refs/heads/master@{#760013}