zdtm: add a test for unsupported children collision

These test is not automatic as after kernel v4.11 behaviour changes, on
older kernel we get children collision:

817 188 0:48 / /zdtm/static/unsupported_children_collision.test/share1 rw,relatime shared:942 - tmpfs share rw
> 818 817 0:124 / /zdtm/static/unsupported_children_collision.test/share1/child rw,relatime shared:943 - tmpfs child1 rw
819 188 0:48 / /zdtm/static/unsupported_children_collision.test/share2 rw,relatime shared:942 - tmpfs share rw
820 819 0:125 / /zdtm/static/unsupported_children_collision.test/share2/child rw,relatime shared:944 - tmpfs child2 rw
> 821 817 0:125 / /zdtm/static/unsupported_children_collision.test/share1/child rw,relatime shared:944 - tmpfs child2 rw

Signed-off-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>