tests: improve read-only test for snapshot

Switch to use ext2 to make it usable on older systems.
Previous test has not been able to catching problem.

Multiple tests are now put in.

FIXME: validate what is doing kernel target when
the header is undeleted and same chunk size is used.

It seems snapshot target successfully resumes and
just complains COW is not big enough:

kernel: dm-8: rw=0, want=40, limit=24
kernel: attempt to access beyond end of device

When chunk size is different it fails instantly.

For checking this with lvm2 and this test case use this patch:

--- a/tools/lvcreate.c
+++ b/tools/lvcreate.c
@@ -769,7 +769,7 @@ static int _read_activation_params(struct
lvcreate_params *lp,
        lp->permission = arg_uint_value(cmd, permission_ARG,
                                        LVM_READ | LVM_WRITE);

-       if (lp->snapshot) {
+       if (0 && lp->snapshot) {
                /* Snapshot has to zero COW header */
                lp->zero = 1;
                lp->wipe_signatures = 0;

---
and switch to use  -c 4 for both snapshots
tests: support for older snapshot target version

TODO: unsure about 1.10...
lvresize: fix stripe size validation

While stripe size is twice the physical extent size,
the original code will not reduce stripe size to maximum
(physical extent size).

Signed-off-by: Zhiqing Zhang <zhangzq.fnst@cn.fujitsu.com>
snapshots: fix incorrect calculation of cow size

Code uses target driver version for better estimation of
max size of COW device for snapshot.

The bug can be tested with this script:
VG=vg1
lvremove -f $VG/origin
set -e
lvcreate -L 2143289344b -n origin $VG
lvcreate -n snap -c 8k -L 2304M -s $VG/origin
dd if=/dev/zero of=/dev/$VG/snap bs=1M count=2044 oflag=direct

The bug happens when these two conditions are met
* origin size is divisible by (chunk_size/16) - so that the last
  metadata area is filled completely
* the miscalculated snapshot metadata size is divisible by extent size -
  so that there is no padding to extent boundary which would otherwise
  save us

Signed-off-by:Mikulas Patocka <mpatocka@redhat.com>