[infra] Shard VM and CFE nnbd builders more

These builders were quite slow:

80min->30min for vm-kernel-nnbd-linux-debug-x64-try
50min->45min for front-end-nnbd-linux-release-x64-try
40min->18min for vm-kernel-nnbd-linux-release-x64-try

Change-Id: I3d772264d82a864a5d278ada2987eadb0cb45bc2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/150983
Reviewed-by: William Hesse <[email protected]>

Author: athomas

tools/bots/test_matrix.json

@@ -1119,37 +1119,41 @@
           ]
         },
         {
-          "name": "weak sdk tests",
+          "name": "weak co19 tests",
           "arguments": [
             "-ncfe-weak-${system}",
-            "corelib",
-            "ffi",
-            "language",
-            "lib"
-          ]
+            "co19"
+          ],
+          "fileset": "front-end",
+          "shards": 1
         },
         {
-          "name": "weak co19 tests",
+          "name": "strong co19 tests",
           "arguments": [
-            "-ncfe-weak-${system}",
+            "-ncfe-strong-${system}",
             "co19"
-          ]
+          ],
+          "fileset": "front-end",
+          "shards": 1
         },
         {
-          "name": "strong sdk tests",
+          "name": "weak sdk tests",
           "arguments": [
-            "-ncfe-strong-${system}",
+            "-ncfe-weak-${system}",
             "corelib",
             "ffi",
             "language",
             "lib"
           ]
         },
         {
-          "name": "strong co19 tests",
+          "name": "strong sdk tests",
           "arguments": [
             "-ncfe-strong-${system}",
-            "co19"
+            "corelib",
+            "ffi",
+            "language",
+            "lib"
           ]
         }
       ]
@@ -1772,7 +1776,9 @@
             "co19",
             "standalone/io",
             "vm"
-          ]
+          ],
+          "fileset": "vm-kernel",
+          "shards": 2
         },
         {
           "name": "vm nnbd tests in strong mode",
@@ -1785,7 +1791,9 @@
             "co19",
             "standalone/io",
             "vm"
-          ]
+          ],
+          "fileset": "vm-kernel",
+          "shards": 2
         }
       ]
     },