tests: add function ts_check_losetup

Unfortunately losetup -f does not return an error nowadays.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: use ts_check_losetup in losetup test

In losetup test we have to handle the special case that ts_check_losetup
requires a working losetup binary.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: ts_device_init requires loop support

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: some more tests need loop support

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

Sort out global and local vars. Now we set TS_DEVICE globally
to be use later.
Skip really quiet if modprobe fails.

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