We intended to do an in-place sort of the mount-point list, but
sorted() returns a new list that wasn't captured. Move to the .sort()
function.
It seems the existing unit-test missed this. Add a new test taken
from the bug which does exhibit a sorting issue. Also added a
unit-test of just the comparitor for sanity.
Closes-Bug: 1699437
Change-Id: I8101e4a1804a4af7dbda20d48bf362c3f4ad2742
ed3c5d9711