[Tests] Add test for small cell

[Cstandardlib]

Linked Issue

Fix #7170
Add tests for #7023, #7036

Unit Tests and/or Case Tests for my changes

• Add test cases for high density cells that may be encountered in SDFT calculations.

[Copilot]

Pull request overview

Adds a new SDFT regression test case targeting very small (high-density) cells to cover the “small cell” scenario discussed in #7170 / #7023 (related to the mxr/Ewald changes in #7036).

Changes:

• Register the new CPU SDFT case in tests/06_SDFT/CASES_CPU.txt.
• Add a new test directory 24_PW_SDFT_SMALL_CELL/ with INPUT, KPT, STRU, result.ref, and a brief README.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/06_SDFT/CASES_CPU.txt	Adds the new 24_PW_SDFT_SMALL_CELL case to the CPU SDFT test list.
tests/06_SDFT/24_PW_SDFT_SMALL_CELL/INPUT	Defines a small-cell SDFT SCF run configuration (PW basis, stress/force enabled).
tests/06_SDFT/24_PW_SDFT_SMALL_CELL/KPT	Provides a small k-point mesh for the case.
tests/06_SDFT/24_PW_SDFT_SMALL_CELL/STRU	Defines the small-cell deuterium structure and pseudopotential reference.
tests/06_SDFT/24_PW_SDFT_SMALL_CELL/result.ref	Adds reference values used by the integration test harness.
tests/06_SDFT/24_PW_SDFT_SMALL_CELL/README	One-line description of the test purpose.

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