[cmake] Declare renderer backend as public (#3717)

test/CMakeLists.txt

@@ -148,10 +148,6 @@ if(MLN_WITH_OPENGL)
             ${PROJECT_SOURCE_DIR}/test/renderer/backend_scope.test.cpp
             ${PROJECT_SOURCE_DIR}/test/util/offscreen_texture.test.cpp
     )
-    target_compile_definitions(
-        mbgl-test
-        PRIVATE MLN_RENDER_BACKEND_OPENGL=1
-    )
 endif()
 
 
@@ -164,17 +160,6 @@ if(MLN_WITH_VULKAN)
             ${PROJECT_SOURCE_DIR}/test/renderer/backend_scope.test.cpp
             ${PROJECT_SOURCE_DIR}/test/util/offscreen_texture.test.cpp
     )
-    target_compile_definitions(
-        mbgl-test
-        PRIVATE MLN_RENDER_BACKEND_VULKAN=1
-    )
-endif()
-
-if(MLN_WITH_METAL)
-    target_compile_definitions(
-        mbgl-test
-        PRIVATE MLN_RENDER_BACKEND_METAL=1
-    )
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL Android)