ZTS: zpool_resilver_restart

Since the vdev initialize feature was integrated the ZTS
zpool_resilver_restart test has been hitting its internal
timeout more frequently. This happens most often on
the coverage builder but not exclusively. Increasing the
timeout for this test case prevents any false positives.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #8273