Fix build error because of missing include directory

tests/CMakeLists.txt

@@ -5,6 +5,7 @@ include_directories(${PROJECT_SOURCE_DIR}/src/server)
 include_directories(${PROJECT_SOURCE_DIR}/plugins)
 include_directories(${PROJECT_BINARY_DIR}/src_generated)
 include_directories(${CHECK_INCLUDE_DIRS})
+include_directories(${PROJECT_SOURCE_DIR}/tests/testing-plugins)
 
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 # Compiled binaries folders #