Use shorten_echo for wrap_tests

We have certain tests which generate extremely long command lines, which
are shortened in the testsuite output with the 'shorten_echo' function.
Currently that is used in run_fdtput_test and run_wrap_test, this patch
uses it for run_wrap_test as well, allowing more general tests with long
command lines.

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