[testing] allow configurable IO chunk size
[testing] allow configurable IO chunk size

This allows the IO chunk size to be configured for the BEL Script
parser. It enables testing how the chunk size may impact parsing.

OpenBEL/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,nbargnesi/bel.rb,nbargnesi/bel.rb,OpenBEL/bel.rb,OpenBEL/bel.rb