[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
 Keywords:          |  Launchpad Bug:
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 I'd like to clean up our unit tests.. there are a lot of archaic idioms
 that have crept in (partly because I'm not up-to-date on modern practices,
 partly because the tests were initially written before those practices
 were established). Here are some things that come to mind:

 * use [https://github.com/testing-cabal/mock "mock"] (which is kinda
 official now, added to stdlib as of py3.3) to shrink our 32 "Fake*"
 classes.
 * replace our home-grown `self.shouldFail` with
 `t.trial.unittest.assertFailure`
 * switch to `inlineCallbacks` to flatten the control-flow nest of helper
 functions

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Ticket URL: <https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2777>
Tahoe-LAFS <https://Tahoe-LAFS.org>
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