dtc: Remove some small bashisms from test scripts

[Jon Loeliger]

> Some of the helper scripts used to run testcases contain some
> constructs that are bashisms.  Or at least which don't work on dash,
> the minimal shell used as /bin/sh on recent Ubuntu systems.
> 
> This patch removes these constructs so that the testsuite will pass
> "out of the box" on systems where /bin/sh is dash.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

jdl