Refactor to avoid code repetition in psitest_library.jl

[GabrielKS]

The psitest_library.jl file is rather repetitive. In particular, the handling of the add_forecasts and other flags is very similar across different systems, sometimes involving identical blocks of code.

[GabrielKS]

As a concrete example, the exact block

for t in 1:2
    ini_time = TimeSeries.timestamp(load_timeseries_DA[t][ix])[1]
    forecast_data[ini_time] = load_timeseries_DA[t][ix]
end

appears fourteen times, plus another seven for the same thing without prefixing timestamp.

[GabrielKS]

Another concrete example: search for PSY.set_variable_cost!.

[GabrielKS]

ini_time = TimeSeries.timestamp(ORDC_cost_ts[t])[1]