tests: add function ts_check_losetup

Unfortunately losetup -f does not returns 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.

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
tests: ts_device_init requires loop support
tests: some more tests need loop support
tests: introduce TS_CMD_UUIDGEN (never use installed one)
tests: skip ts_scsi_debug_init really quiet
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
DEVNAME which were still globally set in that function.
travis-ci: minor cleanup and speedup

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>
travis-ci: enable root checks
debug travis losetup