ZTS: Fix zpool_status_008_pos false positive

Increase the injected delay to 1000ms and the ZIO_SLOW_IO_MS threshold
to 750ms to avoid false positives due to unrelated slow IOs which may
occur in the CI environment. Additionally, clear the fault injection as
soon as it is no longer required for the test case.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16769