tests: Harmonize header use (#25616)

* tests: Harmonize the use of private ggml includes

* tests: In test-backend-ops, use quoted includes

As with all other tests. This is to ensure that the build uses shipped
headers over possibly system-installed ones.

tests/CMakeLists.txt

@@ -239,7 +239,6 @@ if (NOT LLAMA_SANITIZE_ADDRESS AND NOT GGML_SCHED_NO_REALLOC)
   # TODO: repair known memory leaks
   llama_build_and_test(test-opt.cpp)
 endif()
-llama_build_and_test(test-gguf.cpp)
 llama_build_and_test(test-backend-ops.cpp)
 
 llama_build_and_test(test-model-load-cancel.cpp LABEL "model")
@@ -299,11 +298,15 @@ get_filename_component(TEST_TARGET test-c.c NAME_WE)
 add_executable(${TEST_TARGET} test-c.c)
 target_link_libraries(${TEST_TARGET} PRIVATE llama)
 
-llama_build_and_test(test-alloc.cpp)
-target_include_directories(test-alloc PRIVATE ${PROJECT_SOURCE_DIR}/ggml/src)
+if (NOT LLAMA_USE_SYSTEM_GGML)
+    # Needs non-public ggml-impl.h
+    llama_build_and_test(test-gguf.cpp)
+
+    # Needs non-public ggml{,-backend}-impl.h
+    llama_build_and_test(test-alloc.cpp)
+endif()
 
 llama_build(test-export-graph-ops.cpp)
-target_include_directories(test-export-graph-ops PRIVATE ${PROJECT_SOURCE_DIR}/ggml/src)
 if (TARGET gguf-model-data)
     target_link_libraries(test-export-graph-ops PRIVATE gguf-model-data)
     target_compile_definitions(test-export-graph-ops PRIVATE LLAMA_HF_FETCH)