test: Update test-imagetools.sh to match new syntax

The syntax of dumpimage was simplified in commit 12b83187 ("tools:
dumpimage: Simplify arguments"), but the test
(test/image/test-imagetools.sh) was not updated and is now failing.

Update the test to use the new syntax.

Reported-by: Vagrant Cascadian <vagrant@debian.org>
Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
Tested-by: Vagrant Cascadian <vagrant@debian.org>