[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):

 Glyph also pointed me at newer assertion methods, like:

 * `f = self.failureResultOf(d, *expectedExceptionTypes)`: passes iff the
 Deferred has already errbacked, with one of the given types
 * `res = self.successResultOf(d)`: passes iff the Deferred has already
 callbacked
 * `self.assertNoResult(d)`: passes iff the Deferred has not been fired yet

 The general pattern is to avoid waiting for Deferreds as much as possible.
 You call the method that returns a Deferred (with mocks in place for
 everything it might wait upon), then you manually trigger those mocks.
 Before, during, and after the triggers, you keep inspecting the Deferred
 to make sure it's in the right state.

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2777#comment:3>
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