LU-18392 tests: watch destroys_in_flight recovery_small/160

LU-18392 tests: watch destroys_in_flight recovery_small/160

In recovery_small/160, we sleep and check for destroys_in_flight
and make sure the number is low, which indicates destroys are not
blocked.

However there's a 10s timeout, the destroy rpcs could be retried
and before they got put back on error_list again, the destroys_in_flight
could be bumped up, if the test case happen to check destroys_in_flight
during this window the test case could fail.

Use wait_update_cond to watch for the expected drop.

Change-Id: I0b29a90e4c78e80a0b5a522d57ed97db1b698364
Test-Parameters: trivial testlist=recovery-small env=ONLY=160,ONLY_REPEAT=100
Fixes: 27f787daa7 ("LU-15737 ofd: don't block destroys")
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58096
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>