tests: ts_scsi_debug_init must not run in a subshell

ts_skip and ts_die won't work from subshell. Now we simply use
TS_DEVICE which is globally set in that function.

I've made sure that we never change TS_DEVICE variable after
we've got it. So we could use it again for cleanup on exit in
ts_{finalize,die,skip} functions.

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