Add failing reference test

[passaro]

Description of change

One of the property-based tests found a new case where the reference model and Mountpoint implementation diverge. This change adds the case as a failing test:

---- reftests::harness::mutations::regression_put_nested_over_open_file stdout ----
thread 'reftests::harness::mutations::regression_put_nested_over_open_file' panicked at mountpoint-s3/tests/reftests/harness.rs:640:29:
assertion `left == right` failed: for file "a" expecting RegularFile found Directory
  left: Directory
 right: RegularFile

The issue seems to be around the shadowing behavior when a "local" file is open (i.e. not uploaded yet) and a prefix with the same name is created on the bucket. To be further investigated.

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