Improve batch tests runner script

Improve batch tests runner script

The following improvements have been incorporated:
- tweaked hackbench invocation by reducing the number of processes to 72
which is arbitrary but enough to generate a load of 25-30 that better
mimics the real working environment of a heavily loaded system
- parametrized calculating delays between sent messages depending on
the message size for libdbus peer to peer to make it consistent with
other IPC tests.

Change-Id: I5b4da9571286ee87f5a06374d75739a97427b870