common/config: export TEST_DEV for mkfs.xfs

common/config: export TEST_DEV for mkfs.xfs

As of xfsprogs commit 6e0ed3d1 ("mkfs: stop allowing tiny filesystems")
attempts to create XFS filesystems sized under 300M fail, unless
TEST_DIR, TEST_DEV and QA_CHECK_FS environment variables are exported
(or a --unsupported mkfs parameter is provided).

TEST_DIR and QA_CHECK_FS are already exported, while TEST_DEV may only
be locally set if provided via e.g. configs/$HOSTNAME.config. Explicitly
export TEST_DEV to ensure that tests which call _scratch_mkfs_sized()
with an fssize under 300M run normally.

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