test-mountpoint-util: run test in private mount namespace

test-mountpoint-util: run test in private mount namespace

This creates a private mount namespace for test-mountpint-util, with all
propagation from the host turned off. This gives us the guarantee that
/proc/self/mountinfo remains fixed and constant while we operate,
removing potential races against other unrelated stuff running on the
system that changes the mount table.

Prompted-by: #17050
(I doubt this actually fixes 17050, this is mostly to make sure that we
aren't possibly affected by such races in our test)