veyron/security: Fix flaky test.
veyron/security: Fix flaky test.

Flakiness was caused by flakiness of IncorrectPasswordError in standard
library crypto/x509. Solution is to just compare the returned error to nil.
Will notify go folks.

Change-Id: I9f7430bd8868eb7b9bf9253ae35efc9454c88499

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref