qa/standalone: osd-backfill-space.sh

qa/standalone: osd-backfill-space.sh

Problem:
In TEST_backfill_test_sametarget
test fails to keep 1 PG at backfill-toofull
and 1 PG at active+clean due to not giving enough
time from 1 backfill to the other

In TEST_ec_backfill_multi
Similar to the test_sametarget above, not enough
time from 1 backfill to the other

Solution:
Give more time gap between one backfill request
from another using sleep <duration> bash command

Fixes: https://tracker.ceph.com/issues/74524
Signed-off-by: Kamoltat (Junior) Sirivadhna <ksirivad@redhat.com>