tests: make inotify-rotate more robust and efficient

tests: make inotify-rotate more robust and efficient

* tests/tail-2/inotify-rotate.sh: Use retry_delay_
to employ an exponential backoff with a total delay of
up to 25.5s.  The 15s delay was seen to trigger a false
failure in http://hydra.nixos.org/build/16546517
Also remove the .1s sleep in each of the 50 iterations
to reduce the running time of the test and thus the
expensive_ tag on this test was removed.
Also ensure that we use the standard exit procedure
upon failure to avoid any erroneous diagnostics due
to persistent files on NFS.