BIND 10 #2395: introduce a simpler way of using waked data source from auth tests

[BIND 10 Development]

#2395: introduce a simpler way of using waked data source from auth tests
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            Reporter:  jinmei    |                        Owner:
                Type:  defect    |                       Status:  new
            Priority:  medium    |                    Milestone:  New Tasks
           Component:  b10-auth  |                     Keywords:
           Sensitive:  0         |              Defect Severity:  N/A
         Sub-Project:  DNS       |  Feature Depending on Ticket:
Estimated Difficulty:  0         |          Add Hours to Ticket:  0
         Total Hours:  0         |                    Internal?:  0
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 Some of the current tests in auth_srv_unittest tweaks existing
 data source clients by bruteforcing its contents (using the
 `getDataSources()` interface and directly rewrites the internal).
 IMO it relies too much on the internal of data source clients and
 makes both the class and tests fragile.  For tests of client-list
 itself it might be considered a necessary evil, but for tests of their
 users I think it goes too far.

 This also one major reason we currently need (in #2211, hopefully soon
 in master) the `swapDataSrcClientLists`, which also makes the
 `DataSrcClientsMgr` integrity weaker.  It's also the reason why we
 need to return a shared pointer from
 `DataSrcClientsMgr::Holder.findClientList()` (because this test
 internally destroys the original due to the brutteforce replacement)
 even though it could return a bare pointer based on its nature.

 What we actually need is, IMO, a cleaner way for tests like
 `AuthSrvTest` to use some faked data source clients (e.. one that
 throws on `findZone()`, instead of relying on the low level internal
 details of the client list implementation.

 This task is to design such a way and implement it.

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Ticket URL: <http://bind10.isc.org/ticket/2395>
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