Move ziltest.sh to the ZTS framework

Move ziltest.sh to the ZTS framework

The ziltest.sh script is a test case designed to verify the correct
functioning of the ZIL.  For historical reasons it was never added
to the test suite and was always run independantly.

This change rectifies that.  The existing ziltest.sh has been
translated in to `slog_015_pos.ksh` and added to the existing
slog test cases.

Reviewed-by: Don Brady <don.brady@intel.com>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5758