Wait for resilver after online

This test performs a rapid offline/online cycle of each of several
mirror vdevs. It can run so quickly that there isn't sufficient pool
redundancy to perform an offline. The solution is to wait until the
pool is resilvered following the online operation.

Also, add a pool sync before the offline operation to help reduce
spurious errors.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #6900