move unit tests out of backend_test.in.cpp (#1880)

test/CMakeLists.txt

@@ -100,6 +100,7 @@ set(MULTI_TEST_SRC
     backend_comparison.in.cpp
     backend_dot.in.cpp
     backend_one_hot.in.cpp
+    backend_pool.in.cpp
     backend_reduce.in.cpp
     backend_reshape.in.cpp
     backend_sum.in.cpp