Factor signal checks out of test scripts

Several test scripts now have some code to check for a program returning
a signal, and reporting a suitable failure.  This patch moves this
duplicated code into a helper function in tests.sh.  At the same time we
remove a bashism found in the current copies (using the non portablr $[ ]
construct for arithmetic).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>