fdtget-runtest.sh: Fix failures when /bin/sh isn't bash

On Ubuntu, /bin/sh is dash (at least by default), and dash's echo doesn't
accept the -e option. This means that fdtget-runtest.sh's EXPECT file will
contain "-e foo" rather than just "foo", which causes a test failure.

To work around this, run /bin/echo instead of (builtin) echo, which has
more chance of supporting the -e option.

Another possible fix is to change all the #! lines to /bin/bash rather
than /bin/sh, and change run_tests.sh to invoke sub-scripts using $SHELL
instead of just "sh". However, that would require bash specifically, which
may not be desirable.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
Acked-by: David Gibson <david@gibson.dropbear.id.au>