LU-11085 tests: Add performance test for ldlm_extent code

LU-11085 tests: Add performance test for ldlm_extent code

Add a new test module "ldlm_extent" which exercises the extent code by
creating multiple extent locks, and discarding them.
Each run is timed and a number of runs are combined to provide a
mean and standard deviation.

Two different tests are performed, with a ramp of locks to keep to
allow seeing any scalability issues:
1/ create lots of non-overlapping extents in
   random order, keeping up to 8000 at a time.
2/ create both random tiny extents and whole-file
   extents, alternating.  Keep up to 1,000,000.
   These are PR and so don't conflict.

Each test runs for at most 5 minutes
(30 loops of 10 seconds each = 300 seconds).

Test-Parameters: trivial env=SLOW=yes env=ONLY=842 testlist=sanity
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I552da3c64fb467cbefb7d25eee709dd038bd454f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54204
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>