add a negative test for flavor_extra_specs

When unseting a nonexistent key, an exception should be raised.
when settting flavor extra specs, key is arbitrary, so
'set_nonexistent_key' does not make sense

Change-Id: I1b94a8b9524ed67dfc3fb1afd51442b740873638
diff --git a/tempest/api/compute/admin/test_flavors_extra_specs.py b/tempest/api/compute/admin/test_flavors_extra_specs.py
index ce326a3..fb6a463 100644
--- a/tempest/api/compute/admin/test_flavors_extra_specs.py
+++ b/tempest/api/compute/admin/test_flavors_extra_specs.py
@@ -115,6 +115,14 @@
                           self.flavor['id'],
                           'key1')
 
+    @attr(type=['negative', 'gate'])
+    def test_flavor_unset_nonexistent_key(self):
+        nonexistent_key = rand_name('flavor_key')
+        self.assertRaises(exceptions.NotFound,
+                          self.client.unset_flavor_extra_spec,
+                          self.flavor['id'],
+                          nonexistent_key)
+
 
 class FlavorsExtraSpecsTestXML(FlavorsExtraSpecsTestJSON):
     _interface = 'xml'