Add tests to assert filter validation

Add test cases to pass valid and invalid filters to server.
If a valid filter is passed, assert server returns the correct
list of resources. If an invalid filter is passed, assert
server returns a 400 response.

Change-Id: I2b8baa93a5587fa036a8a8ab124c216d2dba4444
Related-Bug: #1749820
diff --git a/neutron_tempest_plugin/api/test_subnets.py b/neutron_tempest_plugin/api/test_subnets.py
index b7a1b21..b8842ab 100644
--- a/neutron_tempest_plugin/api/test_subnets.py
+++ b/neutron_tempest_plugin/api/test_subnets.py
@@ -64,7 +64,8 @@
     def test_list_no_pagination_limit_0(self):
         self._test_list_no_pagination_limit_0()
 
-    @decorators.skip_because(bug="1749820")
     @decorators.idempotent_id('c0f9280b-9d81-4728-a967-6be22659d4c8')
     def test_list_validation_filters(self):
-        self._test_list_validation_filters()
+        self._test_list_validation_filters(self.list_kwargs)
+        self._test_list_validation_filters({
+            'unknown_filter': 'value'}, filter_is_valid=False)