tests: Better handling of valgrind errors saving blobs

Currently we have 3 valgrind suppression files in the tests, all of which
are to handle memcheck errors that originate from saving entire buffers
containing blobs where the gaps between sub-blocks might not be
initialized.

We can more simply suppress those errors by having the save_blob() helper
use valgrind's client interface to mark the data as initialized before we
write it out.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>