ffspart: tests require pflash

[shenki]

In order to write good tests, reading the generated file is required (see #202).

Ideas on how to make a pflash available for the test harness are welcome.

We could install it from the distro, as the containers we do CI in ship it in the opal-utils package. If we do that we need to ensure that other tests (eg. pflash ones) use the locally built binary and not the one in the system path.