[test_nhg.py] Cleanup test resources at the end (sonic-net#2128)

*Removed some leftover state from a previous test case that was sometimes messing up with a following test case.
Signed-off-by: Alexandru Banu <v-albanu@microsoft.com>

Author: abanu-ms

tests/test_nhg.py

@@ -1931,6 +1931,9 @@ def create_cbf_invalid_nhg_map_test():
             time.sleep(1)
             assert(not self.nhg_exists('cbfgroup3'))
 
+            # Cleanup
+            self.cbf_nhg_ps._del('cbfgroup3')
+
         self.init_test(dvs, 4)
 
         mainline_cbf_nhg_test()