fusefs tests: Use memcpy to work around a -Wstrlcpy-strlcast-size warning

[jhb]

Summary

tests/sys/fs/fusefs/xattr.cc:572:50: error: size argument in 'strlcpy' call appears to be size of the source; expected the size of the destination [-Werror,-Wstrlcpy-strlcat-size]

572 |                         strlcpy((char*)out.body.bytes, attrs1, sizeof(attrs1));
    |                                                                ~~~~~~~^~~~~~~

The warning is correct in that the size is the size of the source, but
that is intended in this case. Use memcpy() instead of strlcpy() to
match the same code in the size_only_race_smaller test above.

[jhb]

I don't know why, but I got the new error after applying the change in D49787 (which makes no sense to me, I would have expected to have seen this error before regardless).

[jhb]

Hmm, I'm getting this on stock main it seems so will push it to fix the build.