Add testfile with varying forecast period.

Wanted for new documentation of PP loading mechanisms.
I just wanted some data where forecast_period varies (but without hybrid levels, so printed form looks  less complex).
Data is actually a small chunk out of ```iris.tests.get_data_path(('PP', 'ukV1', 'ukVpmslont.pp'))```,
which loads as :
```
air_pressure_at_sea_level / (Pa)    (time: 3; grid_latitude: 928; grid_longitude: 744)
     Dimension coordinates:
          time                           x                 -                    -
          grid_latitude                  -                 x                    -
          grid_longitude                 -                 -                    x
     Auxiliary coordinates:
          forecast_period                x                 -                    -
     Scalar coordinates:
          forecast_reference_time: 2010-02-08 03:00:00
     Attributes:
          STASH: m01s16i222
          source: Data from Met Office Unified Model 7.03
```