[tahoe-lafs-trac-stream] [Tahoe-LAFS] #3917: Refactor `UtilTests` class

[Tahoe-LAFS]

#3917: Refactor `UtilTests` class
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 Reporter:  fenn-cs  |          Owner:
     Type:  defect   |         Status:  new
 Priority:  normal   |      Milestone:  undecided
Component:  unknown  |        Version:  n/a
 Keywords:           |  Launchpad Bug:
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 Update test such as `UtilTests` in src/allmydata/test/test_storage.py  to
 either use `SyncTestCase` or `AsyncTestCase` base test case and not
 `unittest.TestCase`

 This refactor would also involve using new and better assertion methods.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/3917>
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