tests: introduce TS_LOOP_DEVS for cleanup

tests: introduce TS_LOOP_DEVS for cleanup

We are maintaining an array TS_LOOP_DEVS to de-initialize devices
always on exit. Until now there was no cleanup in ts_skip().

The downside is that we can't execute ts_device_init() in a subshell
anymore. The device is returned via global variable TS_LODEV, similar
like we do already in ts_scsi_debug_init().

Tests which don't use ts_device_init() to create loop devices may use
ts_register_loop_device() to get them cleaned up later.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>