Fix up concurrent test zpool setup and teardown

Set up zpools with a more unique name, stash the zpool name away in a file pointed
to by $ZPOOL_NAME_FILE (which is relative to a per-testcase generated temporary
directory), then remove the file based on $ZPOOL_NAME_FILE in the cleanup
routines.

This is a more concurrency-safe solution and will allow the testcases to be safely
executed in parallel.

Reviewed by: kevans, jtl
Approved by: jtl (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D19024