check: put temporary files in TMPDIR, not /tmp

check: put temporary files in TMPDIR, not /tmp

Nowadays, systemd will auto-remove files from /tmp after 10 days.  If
you want to run a testcase for more than that duration (e.g.
SOAK_DURATION=14d) then the test will fail after the .out file is
deleted:

 xfs/286            _check_xfs_filesystem: filesystem on /dev/sda4 is inconsistent (r)
 (see /var/tmp/fstests/xfs/286.full for details)
 sed: can't read /tmp/2098.out: No such file or directory
 - output mismatch (see /var/tmp/fstests/xfs/286.out.bad)
 mv: cannot stat '/tmp/2098.out': No such file or directory
 diff: /var/tmp/fstests/xfs/286.out.bad: No such file or directory

This happens because systemd-tmpfiles garbage collects any file in /tmp
that becomes older than 10 days:

 $ cat /usr/lib/tmpfiles.d/tmp.conf
 #  This file is part of systemd.
 #
 #  systemd is free software; you can redistribute it and/or modify it
 #  under the terms of the GNU Lesser General Public License as published by
 #  the Free Software Foundation; either version 2.1 of the License, or
 #  (at your option) any later version.

 # See tmpfiles.d(5) for details.

 # Clear tmp directories separately, to make them easier to override
 q /tmp 1777 root root 10d
 q /var/tmp 1777 root root 30d

This is now the default in Debian 13 (D12 never deleted anything) which
is why I didn't notice this until I upgraded a couple of weeks ago.
Most people aren't going to be running a single testcase for more than
10 days so I'll go with the least invasive solution that I can think of.

Allow system administrators or fstests runners to set TMPDIR to a
directory that won't get purged, and make fstests follow that.  Fix up
generic/002 so that it doesn't use $tmp for paths on the test
filesystem.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>