FF table count test data.

Update test data to contain a suitable file for testing the FF fixed header lookup table `table_count` entry.

This particular file contains 4 legitimate fields plus 1 empty field. 

The original `n48_multi_field` file tests logic in `iris.fileformats.ff.FF2PP._extract_field` to detect when an empty field in the lookup table is encountered and terminates reading. Note that, the associated FF fixed header lookup table `table_count` entry is set to 5.

This variant has identical content, but has the FF fixed header lookup table `table_count` entry set to 4, to match the number of legitimate fields contained within the file.