test: check for /dev/loop-control when checking lodev availability

test: check for /dev/loop-control when checking lodev availability

losetup in util-linux 2.40 started reporting lost loop devices [0] and
it has an unfortunate side-effect where it reports lost devices even in
containers, which then makes the loop device check "falsely" pass [1].

Let's just check for /dev/loop-control explicitly to "work around" this.

[0] https://github.com/util-linux/util-linux/commit/a6ca0456cc6d704a786f6b66d8bb2d89ff18eba7
[1] https://github.com/util-linux/util-linux/issues/2824