ZTS: Retry in import_rewind_config_changed.ksh

As explained by the disclaimer in the test case,

"This test can fail since nothing guarantees that old
MOS blocks aren't overwritten."

This behavior is expected and correct, but results in a
flaky test case which is problematic for the CI. The best
we can do to resolve this is to retry the sub-test which
failed when the MOS blocks have clearly been overwritten.

When testing failures were rare enough that a single retry
should normally be sufficient. However, we allow up to
five for good measure.

Reviewed by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13119