dtc: Remove some small bashisms from test scripts

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@gibson.dropbear.id.au>