Adding -b (buffer size) parameter to varnishtest

[Dridi Boukelmoune]

> At commit 1628f0b, there are 469 test cases. 34 produce more than 50KB log.
> Of those 34, there are 7 that use more than 100KB:

I think it is interesting for VMOD writers or even Varnish users that
want to test their VCL+backends to be able to raise this value.
Especially when you want to reproduce a regression that may involve
many steps.

Dridi