xfstests: check if qa user can execute commands

xfstests: check if qa user can execute commands

_require_user only checks if the qa user exists in /etc/passwd. It may exist
though still not be able to execute anything (e.g. with a /bin/false shell).

_user_do cannot determine failure to execute the given command, as it uses
_user_do_filter for post-processing which succeeds even if the command
fails. Thus, the check should be performed by _require_user.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Christoph Hellwig <hch@lst.de>