[infra] Increase shards for co19 tests on VM AOT NNBD builder

With more co19 tests getting migrated, some shards were timing out.

Change-Id: I2e8acbf6f41cb472fc48a8bcdbf07d2387623ad1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156024
Reviewed-by: Karl Klose <karlklose@google.com>

Author: athomas

tools/bots/test_matrix.json

@@ -1316,7 +1316,7 @@
             "co19"
           ],
           "fileset": "vm-kernel",
-          "shards": 2
+          "shards": 3
         },
         {
           "name": "co19 nnbd tests in strong mode",
@@ -1325,7 +1325,7 @@
             "co19"
           ],
           "fileset": "vm-kernel",
-          "shards": 2
+          "shards": 3
         },
         {
           "name": "vm nnbd tests in weak mode with asserts",