TransformFstabForDsu: Insert synthesised entry at end of mount group

The intention of inserting this synthesised mount entry is for the
FirstStageMount() procedure to eventually fallback to this entry if all
previous mount lines failed.
In order for FirstStageMount() to retry mount, the mount lines have to
be grouped together.
This change ensures that would happen and add test.

Bug: 220074274
Test: atest CtsFsMgrTestCases
Change-Id: Id042a6a6738d27c06a397ef7f4e0977907371c05