Use `HttpNotImplemented` in `tests.v3.test_trusts`

Since we are moved to usage exceptions from oslo, `HTTPNotImplemented`
was renamed to `HttpNotImplemented`, so we need to change usage of this
exception in tests.

Related to blueprint common-client-library-2

Change-Id: I51f6593ebb86267dad7086d4862a135743f2de7f
Merge "Use `HttpNotImplemented` in `tests.v3.test_trusts`"