[FragmentItem] Enable FragmentItem for tests

This patch changes the status of |LayoutNGFragmentItem| to
|test|.

Bug: 982194
Change-Id: Ie98a1c454a0a6e423a6ff19c17a783434f9c55f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1782019Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781975}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -951,6 +951,7 @@
     {
       name: "LayoutNGFragmentItem",
       implied_by: ["LayoutNGBlockFragmentation", "LayoutNGFragmentTraversal"],
+      status: "test",
     },
     {
       // Traverse the fragment tree when painting and hit-testing, instead of