cppgc: Disable flaky weak container test

The test WeakContainerTest.ConservativeGCTracesWeakContainer failed on
one of the bots:
https://ci.chromium.org/p/v8/builders/ci/V8%20Mac64%20ASAN/29755?

This seems like flakiness related to stack scanning.

Bug: v8:11052
Change-Id: Ia60d64e63a071335c4c3f648a4ba0c1823841617
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2492324
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Auto-Submit: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70715}

test/unittests/unittests.status

@@ -4,6 +4,10 @@
 
 [
 ##############################################################################
+[ALWAYS, {
+  'WeakContainerTest.ConservativeGCTracesWeakContainer': [SKIP],  # test is flaky.
+}],
+
 ['system == macos and asan', {
   # BUG(820416).
   'BitsDeathTest*': [SKIP],
@@ -76,5 +80,4 @@
 ['variant == stress_snapshot', {
   '*': [SKIP],  # only relevant for mjsunit tests.
 }],
-
 ]