test-meta: use grep -a on xstat/meta

test-meta: use grep -a on xstat/meta

xattr data may contain arbitrary bytes, including NUL.
If we use grep on such data without -a, then it just
prints "Binary file (standard input) matches" and we
haven't tested anything.

Use grep -a for the appropriate invocations to avoid
this issue, we only care about ascii lines here.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>