Refactor expirer unit tests
Refactor expirer unit tests

In expirer's unit tests, fake InternalClient classes are defined
and its instance simulates expirer's task queue behaviors.

To make review for expirer's task queue update patch [1] easy,
this patch refactors the implementation of the fake InternalClient
classes. In this patch, unit tests are refactored by the following
two approaches:
    #1: Summarizing duplicated fake InternalClient implementation
    #2: Make task account name variable
The #2 approach is for multiple task accounts in [1].

The patch [1] will be rebased after this patch merged.

[1]: https://review.openstack.org/#/c/517389

Change-Id: I10a7151cfdd43460ad38c47f672d3c31b77e7990

tipabu/swift,tipabu/swift,swiftstack/swift,smerritt/swift,matthewoliver/swift,openstack/swift,tipabu/swift,smerritt/swift,matthewoliver/swift,matthewoliver/swift,swiftstack/swift,smerritt/swift,smerritt/swift,openstack/swift,tipabu/swift,openstack/swift,openstack/swift,matthewoliver/swift,swiftstack/swift