[test] Bump shards on slow optional trybot

Also skip the slowest test.

TBR=sergiyb@chromium.org
NOTRY=true

Change-Id: I9646dc750fafe47a0680e57ed029ab24a521d1a3
Reviewed-on: https://chromium-review.googlesource.com/c/1273885Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56535}

infra/testing/builders.pyl

@@ -332,7 +332,7 @@
   },
   'v8_linux_arm64_gc_stress_dbg': {
     'tests': [
-      {'name': 'd8testing', 'test_args': ['--gc-stress'], 'shards': 8},
+      {'name': 'd8testing', 'test_args': ['--gc-stress'], 'shards': 10},
     ],
   },
   'v8_linux_arm64_rel_ng_triggered': {

test/mjsunit/mjsunit.status

@@ -799,6 +799,9 @@
   'es6/classes': [PASS, ['tsan', SKIP]],
   'regress/regress-1122': [PASS, ['tsan', SKIP]],
 
+  # Too slow with gc_stress on arm64.
+  'messages': [PASS, ['gc_stress and arch == arm64', SKIP]],
+
   # Slow on arm64 simulator: https://crbug.com/v8/7783
   'string-replace-gc': [PASS, ['arch == arm64 and simulator_run', SKIP]],