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/test-backend-ops.cpp

@@ -15,10 +15,10 @@
 // ##############################
 
 
-#include <ggml.h>
-#include <ggml-alloc.h>
-#include <ggml-backend.h>
-#include <ggml-cpp.h>
+#include "ggml.h"
+#include "ggml-alloc.h"
+#include "ggml-backend.h"
+#include "ggml-cpp.h"
 
 #include <algorithm>
 #include <atomic>