MFC r293073:

MFC r293073:

- Use attach_md instead of hardcoding md(4) provider unit numbers
- Implement a gmirror_test_cleanup function, which in turn calls
  geom_test_cleanup to clean up all md(4) providers allocated in the test
  run.
- Remove duplicate logic in test scripts for removing md(4) providers.
- Don't create files in /tmp (outside the kyua sandbox); use the current
  directory instead

git-svn-id: svn://svn.freebsd.org/base/stable/10@294657 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f