xz/lzma warning causes test to fail

xz/lzma warning causes test to fail

OpenBSD's port building infrastructure is able to build in a privilege
separated mode. It uses a privilege drop model. Regression tests fail in
this mode as xz/lzma is unable to set file group and errors out with:

xz: tmp.xz: Cannot set the file group: Operation not permitted
gmake[1]: *** [Makefile:307: zstd-playTests] Error 2

Actually it is not a xz/lzma error but a warning causing zstd's
regression test to fail. Proposed fix is to have xz/lzma not set the
exit status to 2 even if a condition worth a warning was detected (-Q
flag).