Assert there is only one data folder created during test (1/n)

See b/170689774#comment2.

If the assertion fails, it will log the content of the 'after' list
which gives us more information for debugging.

Bug: 170689774
Test: atest StagedRollbackTest
Change-Id: Ib4ccd1be6f39c1d4c2556cbf6a06cfdf14240220