Add TestGrid channel under sig-testing

Author: chases2

communication/slack-config/restrictions.yaml

@@ -56,6 +56,7 @@ restrictions:
       - "^sig-testing"
       - "^kind"
       - "^prow"
+      - "^testgrid"
       - "^testing"
     usergroups:
       - "^test-infra-oncall$"

communication/slack-config/sig-testing/config.yaml

@@ -3,6 +3,7 @@ channels:
   - name: prow
   - name: prow-alerts
   - name: sig-testing
+  - name: testgrid
   - name: testing-commons
     archived: true
-  - name: testing-ops
+  - name: testing-ops