[VM/Testing] - Turn on co19_2 legacy testing in NNBD bots.

Change-Id: I5592a5a8d6dfd068e45ecf9b4ec2d12fbe67b4a2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138621
Commit-Queue: Siva Annamalai <asiva@google.com>
Reviewed-by: Liam Appelbe <liama@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

tools/bots/test_matrix.json

@@ -1301,6 +1301,15 @@
           "fileset": "vm-kernel-nnbd",
           "shards": 6
         },
+        {
+          "name": "vm co19_2 tests",
+          "arguments": [
+            "-ndartkp-weak-asserts-${system}-${mode}-${arch}",
+            "co19_2"
+          ],
+          "fileset": "vm-kernel-nnbd",
+          "shards": 10
+        },
         {
           "name": "vm weak tests with asserts",
           "arguments": [