TrigByteStreamTools: Modernise mergeBSfiles script and add unit test

The mergeBSfiles script was broken as reported in ATR-21808. Fix, modernise, and add a unit test for the script.

Changes:

• use argparse instead of getopt
• use os.listdir to find files
• use logging module for messaging
• encapsulate code in functions
• add new option -N to copy only N events from each input file
• add a unit test