test(dfpt): dedupe repeated analytic blocks in the phon serial test

Share the occupied-weights table, the single-plane-wave psi builder, the
analytic accumulate_electron cross term (now on top of AnalyticDVloc),
and the isotropic loto data setup (eps/Born charges + two-atom mass
table via MakeTwoAtomCell) through phon fixture helpers; the three
AccumulateElectron tests and the two loto closed-form tests keep their
reference formulas but drop the duplicated inline copies.

Author: Zanthoxylum