check: Introduce fallback_device() and cleanup_fallback_device()

check: Introduce fallback_device() and cleanup_fallback_device()

These optional functions can be defined by a test case script. When
defined and TEST_DEVS is empty, the fallback_device() is executed before
runing the test case. The fallback_device() function intializes a virtual
device to run the test case and return the device to be set in TEST_DEVS.
After running the test case, cleanup_fallback_device() is executed to
clean up the device.

This feature allows to run test cases with test_device() function even if
TEST_DEVS is not set in the config, using virtaul devices such as null_blk.

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