lost "#include <atomic>" import in "nested_test.cpp" file

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compiler report a error at line "167" when compile "nested_test.cpp" file.

std::atomic<std::size_t> counting_allocator_base::allocatedCount{0};

by test, found lost "#include < atomic>" import in "nested_test.cpp" file !

Could PR fix it next commit ? @lewissbaker @cor3ntin