tests: Introduce zbd test group

tests: Introduce zbd test group

The zoned block device (zbd) test group is used to gather all tests
specific to zoned block devices (null_blk device with zoned mode enabled,
SMR disks, dm-linear on top of zoned devices, etc). Execution of this group
requires that the kernel be compiled with the block layer
CONFIG_BLK_DEV_ZONED option enabled and also requires the null_blk driver
to have zoned mode support (added in kernel 4.19).

This group rc script implements _fallback_null_blk_zoned() helper function
which initailize a null_blk device with zoned mode. Each of the zbd group
test cases calls it in fallback_device() function. This allows the zbd
group test cases fallback to the null_blk device even if the TEST_DEVS
is empty. With this, all tests scripts can be written by only defining
the test_device() function while allowing operation on both null_blk and
user specified devices.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Masato Suzuki <masato.suzuki@wdc.com>