OpenZFS 8072 - zfs-tests: several test cases incorrectly spell TESTPOOL

OpenZFS 8072 - zfs-tests: several test cases incorrectly spell TESTPOOL

Authored by: Yuri Pankov <yuri.pankov@nexenta.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: John Kennedy <jwk404@gmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/8072
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/56e4733
Closes #6137