btrfs-progs: tests: Introduce misc-tests/008-leaf-crossing-stripes

btrfs-progs: tests: Introduce misc-tests/008-leaf-crossing-stripes

To check is btrfs-convert create bad filesystem with
leaf across stripes.

It is happened in progs version <=v4.1.2, and fixed by patch titled:
btrfs: convert: Avoid allocating metadata extent crossing stripe boundary
which was merged in v4.2.

Notice thar this testcase can not report error in old version of
btrfs-progs, because "btrfs check" can't check this type of error
in those version, but we have another testcase in fsck-tests, to
check is "btrfs check" support this check.

So, the above 2 testcase together can check leaf-crossing-stripes
bug in all versions.

Signed-off-by: Zhao Lei <zhaolei@cn.fujitsu.com>
[renamed and other minor changes]
Signed-off-by: David Sterba <dsterba@suse.com>