SEDI_SEMI incorrectly halves DT_ATMOS in tests/default_vars.sh

[NickSzapiro-NOAA]

Description

Currently in default_vars.sh#L1178:
if [[ ${SEDI_SEMI} = .true. ]]; then export DT_ATMOS=$((DT_ATMOS/2)); fi

Presumably, semi-Lagrangian method should have a longer timestep, not shorter.
This also conflicts with what's in global-workflow config.fcst#L210-L212

export dt_inner=$((DELTIM/2))
export sedi_semi=.true.
if [[ "${sedi_semi}" == .true. ]]; then export dt_inner=${DELTIM} ; fi

In default_vars, maybe the corresponding implementation is:
if [[ ${SEDI_SEMI} = .false. ]]; then export DT_INNER=$((DT_ATMOS/2)); fi

This bug would increase test wallclock for tests calling export_ugwpv1() @dpsarmie @BrianCurtis-NOAA @DeniseWorthen @junwang-noaa @jkbk2004
Any adjusted k_split, n_split we're running should be revisited too