Subnet rbac tests

Add rbac tests for subnet-related network policy actions. This
commit adds coverage for the following policy actions:

    * create_subnet
    * get_subnet
    * update_subnet
    * delete_subnet

Note that policy actions like create_subnet:segment_id and
get_subnet:segment_id have not been included because it is not
clear how to enforce them (same with create_subnet:service_types:
service_types is not even an attribute exposed in the subnet v2.0
documentation).

Change-Id: I982fde3e065cbf966a03754e99dae8c3fe8715b9
diff --git a/releasenotes/notes/subnet-rbac-tests-6d3cf54e39a7b486.yaml b/releasenotes/notes/subnet-rbac-tests-6d3cf54e39a7b486.yaml
new file mode 100644
index 0000000..8d3d22a
--- /dev/null
+++ b/releasenotes/notes/subnet-rbac-tests-6d3cf54e39a7b486.yaml
@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    Add RBAC tests for network subnet endpoints, providing coverage for the
+    following policy actions:
+
+      * create_subnet
+      * get_subnet
+      * update_subnet
+      * delete_subnet