OpenZFS 7503 - zfs-test should tail ::zfs_dbgmsg on test failure

OpenZFS 7503 - zfs-test should tail ::zfs_dbgmsg on test failure

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Approved by: Gordon Ross <gordon.w.ross@gmail.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Porting Notes:
- Enable internal log for DEBUG builds and in zfs-tests.sh.
- callbacks/zfs_dbgmsg.ksh - Dump interal log via kstat.
- callbacks/zfs_dmesg.ksh - Dump dmesg log.
- default.cfg - 'Test Suite Specific Commands' dropped.

OpenZFS-issue: https://www.illumos.org/issues/7503
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/55a1300
Closes #6002