Add unit tests for module_gint

[Copilot]

Adds unit tests for module_gint following the existing test patterns in the repository.

Test Coverage

• gint_helper.h - index3Dto1D, index1Dto3D, pow_int, floor_div, ceil_div
• BigGridInfo - Constructor, coordinate conversions, meshgrid index operations, max_ext_bgrid_num
• UnitCellInfo - Index conversions, coordinate getters, extended unitcell mapping
• LocalCellInfo - Local/global index conversions, boundary checks, meshgrid operations

Files

source/source_lcao/module_gint/
├── CMakeLists.txt              # Added test subdirectory
└── test/
    ├── CMakeLists.txt
    ├── gint_helper_test.cpp
    ├── biggrid_info_test.cpp
    ├── unitcell_info_test.cpp
    └── localcell_info_test.cpp

Tests are enabled when BUILD_TESTING and ENABLE_MPI are set.

Original prompt

帮我为 module_gint 下的代码添加合适的单元测试，单元测试格式可以参考 source 下的其他单元测试代码

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