Avoid dependency on bash

[rsmmr]

There's really no good reason btest needs to require a bash (vs. just standard sh), so we should modify the scripts accordingly.

[ckreibich]

Do you guys think this is still realistic? With the Windows support we've only cemented dependency on Bash further. (@timwoj, fyi).

[timwoj]

I think this is less about requiring bash specifically over just plain old sh than it is about removing the dependency on bash altogether. I imagine the stuff I added to btest that calls bash could call sh just as easily.

[ckreibich]

Alright, this ticket survives for another few years. ;-)