mountinfo: revamp TestMountedBy
mountinfo: revamp TestMountedBy

The way prepareMounts was written was very complicated from the
maintainability perspective, with all the test cases in the same basket.

Rewrite this in a much cleaner way, with individual subtests.

Add some more test cases that were failing before the previous commit.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

moby/sys