layout: Let stretch on flex item cross size stretch to the line (#38526)

We were instead stretching to the containing block, which implied that
the behaviors of a `stretch` size and `stretch` alignment weren't
consistent.

As resolved by the CSSWG, the behavior will now be:
 - If the cross size of the line is known, stretch to the line.
 - Otherwise, stretch to the containing block.

See https://github.com/w3c/csswg-drafts/issues/11784

This aligns us with Blink, which has already shipped this new behavior.

Testing: Improves existing WPT and adds a new test.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -172605,6 +172605,19 @@
        {}
       ]
      ],
+     "align-self-016.html": [
+      "4b2bc6d2efc9f4b1c68ad33961ec97b5a7e7405b",
+      [
+       null,
+       [
+        [
+         "/css/reference/ref-filled-green-200px-square.html",
+         "=="
+        ]
+       ],
+       {}
+      ]
+     ],
      "alignment": {
       "flex-align-baseline-column-rtl-direction.html": [
        "3be57a2b34bf4c68f0041b874fbfdcecd797a20a",