[test] Bump shards on MSAN

The tests are scratching at the overall timeout.

TBR=mslekova@chromium.org
NOTRY=true

Change-Id: I817337eff76dcf36306d7f5ceaaef34eea7886d3
Reviewed-on: https://chromium-review.googlesource.com/c/1276385
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56565}

infra/testing/builders.pyl

@@ -539,7 +539,7 @@
   },
   'V8 Linux - arm64 - sim - MSAN': {
     'tests': [
-      {'name': 'test262', 'shards': 2},
+      {'name': 'test262', 'shards': 3},
       {'name': 'v8testing', 'shards': 4},
     ],
   },