Tests: fix BookEntry naming warning

[coneiric]

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Renames constructed entry objects for clarity, and resolves clang -Wuninitialized warning:

kovri/tests/unit_tests/client/address_book/impl.cc:177:51: warning: variable 'entry' is uninitialized when used within its own initialization [-Wuninitialized]                                                                                                      
      kovri::client::BookEntry entry("kovri.i2p", entry.get_address()));