LU-7429 tests: generate dangling name entry properly

LU-7429 tests: generate dangling name entry properly

There may be some creation after the dangling injection, such as
create ".lustre/lost+found/MDTxxxx" for LFSCK, create some quota
related local files. These creations may reuse the just released
local object/inode that is referenced by the dangling name entry.
That will fail the dangling injection as to the subsequent LFSCK
will not find dangling name entry. So before deleting the target
object for the dangling name entry, remove some other objects to
avoid the target object being reused by some potential creations.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I0e1bb60c1095119e10009fe2a8ce38687e3e7692
Reviewed-on: https://review.whamcloud.com/24763
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>