Replace macro-based header guard with pragma once

Co-authored-by: wavefunction91 <[email protected]>

Author: Copilot

tests/collocation_common.hpp

@@ -22,7 +22,6 @@
 using namespace GauXC;
 
 #define MAX_NPTS_CHECK 67
-#define REF_COLLOCATION_DATA_DEFINED  // Guard for hdf5_test_serialization_impl.hpp
 
 struct ref_collocation_data {
   std::vector<int32_t>              mask;

tests/hdf5_test_serialization_impl.hpp

@@ -12,10 +12,7 @@
 #pragma once
 
 // This file contains implementations for HDF5 serialization of test data structures
-// Must include struct definitions before this file
-#ifndef REF_COLLOCATION_DATA_DEFINED
-#error "collocation_common.hpp must be included before hdf5_test_serialization_impl.hpp"
-#endif
+// Must include collocation_common.hpp before this file to define ref_collocation_data and ref_weights_data
 
 #include "hdf5_test_serialization.hpp"