Run tests on integration test changes (#2373)

Hopefully this will catch situations like
https://github.com/tailscale/tailscale/pull/2372

Signed-off-by: Christine Dodrill <xe@tailscale.com>

.github/workflows/xe-experimental-vm-test.yml

@@ -1,9 +1,9 @@
 name: "integration-vms"
 
 on:
-  # # NOTE(Xe): uncomment this region when testing the test
-  # pull_request:
-  #   branches: [ main ]
+  pull_request:
+    paths:
+      - "tstest/integration/vms/**"
   push:
     branches: [ main ]
   release: