[test] Bump shards on slow builder

TBR=tmrts@chromium.org
NOTRY=true

Change-Id: Ief480c6d9c2326a875a57d04bec5ec65f9ff0fe5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1695479Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62619}

infra/testing/builders.pyl

@@ -1446,12 +1446,14 @@
       {
         'name': 'mozilla',
         'suffix': 'armv8-a',
-        'test_args': ['--extra-flags', '--enable-armv8']
+        'test_args': ['--extra-flags', '--enable-armv8'],
+        'shards': 2,
       },
       {
         'name': 'test262',
         'suffix': 'armv8-a',
-        'test_args': ['--extra-flags', '--enable-armv8']
+        'test_args': ['--extra-flags', '--enable-armv8'],
+        'shards': 2,
       },
       {
         'name': 'v8testing',