Runner scripts: Clean main functions, and auto-detect RX/TX captures

This branch:

• Updates the runner scripts to automatically detect which results from commands are RX vs TX captures
  • Currently based on file name (whether it starts with rx or tx)
  • As a result, changed --save-result flag in slice test runner script to --save-rx and --save-tx - and crucially users no longer need to state the name of the relevant capture command
• Refactors & updates the implementation of the main functions for the runner scripts - in particular:
  • Removes a few old unused functions
  • Moves some checks currently directly implemented in the main function to separate dedicated functions
  • Moves the parsing & transformation of command-line flag values to dedicated callback functions