MFstable/10 r294657:

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