acpi unit-test: extract iasl executable from configuration

The test checked if iasl is installed by running "iasl"
and checking the error output.
It is better to use the iasl executable as appears
in configuration.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
