LU-12747 tests: wait properly for orhpan thread stop

LU-12747 tests: wait properly for orhpan thread stop

Use wait_update_facet() to check if the MDD orphan cleanup thread has
exited, rather than a fixed 5s timeout.  We can hope that most cases
will finish faster than 5s, but don't gratuitously fail if it takes
somewhat longer.  We clearly aren't having a fatal problem here, or
there would be serious failures at cleanup time.

Fixes: fffef5c29e3b ("LU-11418 mdd: delete name if orphan doesn't exist")
Test-Parameters: trivial testlist=sanity envdefinitions=ONLY=811,ONLY_REPEAT=100
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I16b0281a519d47b5b98d495bf17040153c3ebbe5
Reviewed-on: https://review.whamcloud.com/37395
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>