[tahoe-lafs-trac-stream] [Tahoe-LAFS] #2777: modernize tests, use "mock"

[Tahoe-LAFS]

#2777: modernize tests, use "mock"
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     Reporter:  warner  |      Owner:
         Type:  task    |     Status:  new
     Priority:  minor   |  Milestone:  undecided
    Component:  code    |    Version:  1.11.0
   Resolution:          |   Keywords:
Launchpad Bug:          |
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Comment (by warner):

 Oh, and for reference, the signature of assertFailure (when using
 inlineCallbacks) is:

 * `e = yield self.assertFailure(d, *expectedFailures)`: wait for `d` to
 errback, require that it errbacked with one of the given exception types,
 and yield the exception. Fail if `d` is callbacked instead of being
 errbacked.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2777#comment:4>
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