btrfs/142, btrfs/143: fix dmdust device names

btrfs/142, btrfs/143: fix dmdust device names

After commit aaa132777476 ("fstests: per-test dmdust instances") the
dmdust device names are no longer a plain 'dust-test', they now have
a suffix matching '.N', where N is the test's sequence number.
The test cases btrf/142 and btrfs/143 are still using the old device
names, so they fail. Fix this my making them refer to 'dust-test.$seq'
instead of 'dust-test'.

Fixes: aaa132777476 ("fstests: per-test dmdust instances")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>