Stubs for testing pools, tags, and posts

We need those before implementing multiple API support, as responses
have different tags / values depending on the API.

src/libdanbooru/autotests/test_danbooru_pool.cpp

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src/libdanbooru/autotests/test_danbooru_pool.h

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src/libdanbooru/autotests/test_danbooru_post.cpp

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src/libdanbooru/autotests/test_danbooru_post.h

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src/libdanbooru/autotests/test_danbooru_tag.cpp

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src/libdanbooru/autotests/test_danbooru_tag.h

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