Fix tenacious storage tests for swh.model 6

The logic for testing the tenacious storage proxy by interspersing bad
objects with good ones triggers when the object list is larger than 3
items. However, the allowed error rate of 1 failure for a window of 3
objects would only work for lists larger than six objects (putting at
least 2 good objects between each bad object).

swh.model 6 made the directory, revision and release test object lists 3
object long, triggering the buggy code.

Reducing the window size to 2 objects makes the test logic works for
lists of 3 or more objects.