test: Allow saving and restoring the bloblist

test: Allow saving and restoring the bloblist

Tests which create a new bloblist overwrite the existing one in sandbox.
Provide a flag for tests to declare this behaviour. Save and restore the
bloblist pointer so that other tests remain unaffected.

Note that when sandbox is running normally, the bloblist has been
relocated to high in memory. The existing bloblist tests create a new
bloblist low in memory, so they do not conflict.

Correct a build error on coreboot by using accessors for gd->bloblist:
Signed-off-by: Simon Glass <sjg@chromium.org>