ecryptfs: add nixos/tests/ecryptfs.nix
(cherry picked from commit ab6fc29719)
ecryptfs: test bug from #16766
(cherry picked from commit d781bf94c1)
ecryptfs: add test to release (#16910)
Would have caught regression #16766
(cherry picked from commit f76a8fbbac)
ecryptfs: add test to release-combined.nix
(cherry picked from commit de80d0544c)
