tests: Use valgrind client requests for better checking

libfdt is never supposed to access memory outside the the blob, or outside
the sub-blocks within it, even if the blob is badly corrupted.

We can leverage valgrind's client requests to do better testing of this.
This adds a vg_prepare_blob() function which marks just the valid parts of
an fdt blob as properly initialized, explicitly marking the rest as
uninitialized.  This means valgrind should catch any bad accesses.

We add a call to vg_prepare_blob() to load_blob() so that lots of the
existing testcases will benefit from the extra checking.

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