tests: fix grep expressions for devices

tests: fix grep expressions for devices

ts_is_mounted "/dev/loop1" returned true if /dev/loop17 was
mounted. A very annoying source of sporadic failures since
many years. This issue became more visible since running the
checks in parallel, which increases the probability to get
bigger loop device numbers.

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