Fix geli device cleanup

Final cleanup routines shouldn't be called from testcases; it should be called
from the testcase cleanup routine.

Furthermore, geli_test_cleanup should take care of cleaning up geli providers
and the memory disks used for the geli providers. geli_test_cleanup will always
be executed whereas the equivalent logic in geli_test_body, may not have been
executed if the test failed prior to the logic being run.

Prior to this change, the test case was trying to clean up $md twice: once in
at the end of the test case body function, and the other in the cleanup function.
The cleanup function logic was failing because there wasn't anything to clean up
in the cleanup function and the errors weren't being ignored.

This fixes FreeBSD test suite runs after r345864.

PR: 237128
Reviewed by: asomers, pjd
Approved by: emaste (mentor)
MFC with: r345864
Differential Revision: https://reviews.freebsd.org/D19854