[test] increase shard numbers for slow bot

http://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Linux%20-%20arm%20-%20sim%20-%20debug
V8 Linux ARM SIM Debug bot runs into timeouts consistently in "Check - extra"
suite shard#1 after 45 minutes (shard#1 and shard#2 run for 40~ minutes) due to
unbalanced sharding.

This CL increases the shard size for the bot

R=yangguo@chromium.org
CC=​machenbach@chromium.org,sergiyb@chromium.org

Bug: v8:8174
No-Try: true
Change-Id: I47fdc2568ff93fb88eda5121d45b6f9d86e596a5
Reviewed-on: https://chromium-review.googlesource.com/c/1456096
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59392}

infra/testing/builders.pyl

@@ -104,7 +104,7 @@
   },
   'v8_linux_nosnap_rel': {
     'tests': [
-      {'name': 'v8testing', 'variant': 'default', 'shards': 4},
+      {'name': 'v8testing', 'variant': 'default', 'shards': 6},
     ],
   },
   'v8_linux_nosnap_dbg': {
@@ -217,7 +217,7 @@
       {'name': 'mozilla', 'shards': 2},
       {'name': 'test262', 'shards': 2},
       {'name': 'v8testing', 'shards': 8},
-      {'name': 'v8testing', 'variant': 'extra', 'shards': 3},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 6},
       {'name': 'v8testing', 'variant': 'trusted', 'shards': 2},
     ],
   },
@@ -1226,7 +1226,7 @@
       {'name': 'mozilla'},
       {'name': 'test262'},
       {'name': 'v8testing', 'shards': 8},
-      {'name': 'v8testing', 'variant': 'extra', 'shards': 3},
+      {'name': 'v8testing', 'variant': 'extra', 'shards': 4},
       {'name': 'v8testing', 'variant': 'trusted', 'shards': 2},
       # Armv8-a.
       {