[Zanthoxylum]

Summary

DFPTSerialBase (the shared gtest fixture base class and its header,
dfpt_serial_fixture.{h,cpp}) is replaced by a free-function test tool,
following the established module_pw test/test_tool.{h,cpp} pattern:
small declaration-only header + one implementation TU compiled into
every test target.

• dfpt_test_tool.{h,cpp} provides the shared utilities as free
  functions — setup_cell, make_coulomb_atom, make_nc_atom,
  make_two_atom_cell, make_nlcc_two_atom_cell, setup_bases,
  vloc_coulomb, analytic_dvloc, fkey, gkey — plus the shared
  test-world constants (lat0, ecutwfc, rho_mult, cubic_edge).
• Every test file now derives its own fixture directly from
  ::testing::Test, owns its objects (ucell_, pw_rho_, pw_wfc_,
  qlist_, data_, q_cart_, ...) as members and calls exactly the
  utilities it needs. There is no shared fixture base class and no
  fixture header anywhere in the module.
• The rho files keep their file-local RNG (single-TU determinism).
• CMakeLists.txt swaps dfpt_serial_fixture.cpp for
  dfpt_test_tool.cpp in every test target.

Relationship to #8268

Both PRs touch dfpt_serial_fixture.cpp — #8268 modifies SetupBases
inside it (to call the new PW_Basis::init_serial); this PR deletes the
file entirely. Either merge order resolves trivially:

• feat(pw): expose the serial basis assembly as PW_Basis::init_serial #8268 first: this branch rebases (the deletion simply subsumes the
  edit), and setup_bases adopts init_serial as a two-line follow-up
  (the adoption point is marked in dfpt_test_tool.cpp);
• this PR first: feat(pw): expose the serial basis assembly as PW_Basis::init_serial #8268 rebases by dropping its fixture edit — its
  module_pw API part is unaffected.

Verification

cmake --build build-gcc13 --target MODULE_DFPT_pert_serial \
  MODULE_DFPT_rho_serial MODULE_DFPT_phon_serial MODULE_DFPT_q0_serial \
  MODULE_DFPT_xc_analytic_serial MODULE_DFPT_write_dyn_serial -j 20
ctest --test-dir build-gcc13 -R "MODULE_DFPT"      # 10/10 targets Passed

• All six reworked targets rebuilt from deleted binaries (make exit
  code 0 per target).
• Case counts conserved: pert 7 / phon 20 / rho 11 / q0 6 / xc 6 /
  write_dyn 6 (identical case names, none added or lost).
• Full MODULE_DFPT suite: 10/10.

Documentation

No docs change needed: test-only refactor with no INPUT, interface, or
behavior change.

[ABACUS-PR-BOT]

Static review of 68d7f3fd5 found no functional regression in the fixture extraction; two include cleanups are noted below. Builds and tests were not run.

[ABACUS-PR-BOT]

Non-blocking: This declaration-only header uses Matrix3 and Vector3<double> only by reference. Forward-declare them, as you do for the other argument types, and include their definitions in the implementation and fixtures that need them.

[ABACUS-PR-BOT]

Non-blocking: This newly added include block is repeated below the fixture at lines 69–74. Please keep one copy of each header here and remove the analogous duplicates in the other changed phon, pert, q0 and rho test files.