geom tests: Fix cleanup of ATF tests since r341392

r341392 changed common test cleanup routines in a way that allowed them to
be used by TAP tests as well as ATF tests. However, a late change made
during code review resulted in cleanup being broken for ATF tests, which
source geom_subr.sh separately during the body and cleanup phases of the
test. The result was that md(4) devices wouldn't get cleaned up.

MFC after: 2 weeks
X-MFC-With: 341392