feat: add windows test pool for taskgraph-t

I'd like to start running taskgraph tests on Windows and macOS (taskcluster/taskgraph#842). This ought to add a usable Windows worker pool. Unfortunately, when I tried it out by hand I ended up with errors such as:
```
Resource /subscriptions/108d46d5-fe9b-4850-9a7d-8c914aa6c1f0/resourceGroups/rg-central-us-taskgraph-t/providers/Microsoft.Network/virtualNetworks/vn-central-us-taskgraph-t/subnets/sn-central-us-taskgraph-t referenced by resource /subscriptions/108d46d5-fe9b-4850-9a7d-8c914aa6c1f0/resourceGroups/rg-taskcluster-worker-manager-production/providers/Microsoft.Network/networkInterfaces/nic-qf0rebyslglv1wvqkn9d was not found. Please make sure that the referenced resource exists, and that both resources are in the same region.
```

...so I suspect something else will be needed.

Author: bhearsum

worker-pools.yml

@@ -2955,6 +2955,7 @@ pools:
       - pool-group: comm-t
       - pool-group: comm-t
         suffix: ssd
+      - pool-group: taskgraph-t
     config:
       image:
         by-suffix:
@@ -2989,6 +2990,7 @@ pools:
               alpha: 500
               gpu: 600
               default: 100
+          default: 5
       worker-config:
         genericWorker:
           config: