btrfs-progs: Add make testsuite command for export tests

btrfs-progs: Add make testsuite command for export tests

Export the testsuite files to a separate tar.  Since fsck tests depend
on btrfs-corrupt-block, and misc tests depends on both
btrfs-corrupt-block and fssum, so set it as prerequisites for package
commad.

Because, althougth fssum can be generated by source that are all in
tests directory, and has no rely on the btrfs's structure.  But
btrfs-corrupt-block deeply relys on btrfs's structure.  For consistency,
at the present stage, generete the two when create test tar.

Signed-off-by: Gu Jinxiang <gujx@cn.fujitsu.com>
[ applied without changes, the generated tarball will be different from
  the one after the follow up commits ]
Signed-off-by: David Sterba <dsterba@suse.com>