Address weirdness in snippets test bucket generator #694

@dandhlee helped point out a weirdness in the rpo_test in our recent turbo replication change, that was copied over from snippets_test.
Just want to highlight this here to ensure we take a look and also to inquire if someone knows if there's a reason why we did it this way.
As dan pointed out, if the bucket exists, an infinite loop would be created, and so I'm kind of confused about what's happening here.

https://github.com/googleapis/python-storage/blob/main/samples/snippets/snippets_test.py#L106
https://github.com/googleapis/python-storage/blob/main/samples/snippets/snippets_test.py#L121
https://github.com/googleapis/python-storage/blob/main/samples/snippets/snippets_test.py#L154

#622