MFC r309774,r309778,r309779,r309780:

MFC r309774,r309778,r309779,r309780:

r309774:

Only run mdconfig -d -u 3 if /dev/md3 exists on the system

This will prevent "cleanup failures" (exit code != 0 returned) when
tmpfs is not loaded

r309778:

Make test_unmount usable in cleanup subroutines

- Duplicate test_unmount to _test_unmount
- Remove atf_check calls
- Call _test_unmount from test_unmount, checking the exit code
  at the end, and returning it to maintain the test_unmount
  "contract"

r309779:

- Ignore errors from umount
- Use _test_unmount instead of test_unmount in cleanup

r309780:

Use _test_unmount instead of test_unmount in cleanup to avoid
false positives with atf_check when tmpfs is not loaded, etc

git-svn-id: svn://svn.freebsd.org/base/stable/10@313518 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f