add regression test for duplicate note commitment handling in view server

[redshiftzero]

We have had two cases where duplicate note commitments caused crashes / hangs in the view server, in #3945 and #1507. This was last resolved in #4423.

These bugs had privacy impact: they can be used to target a specific user and identify their network location when the attacker notices their view service crash.

As such, it would be wise to add a regression test to prevent re-occurrence of this in the future.