test: optimize unit test .gitignore files

Just add a single .gitignore file in test/unit
that covers *_ut.  That allows us to eliminate
100 .gitignore files in the test/unit directory
hierarchy.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ia190587b4d5c6f1847471be27550cbfb843dc01e

Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/9235


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>