LU-16872 tests: exercise sanity test_27M more fully

LU-16872 tests: exercise sanity test_27M more fully

Improve the sanity.sh test_27M to precreate a bunch of files with
specific OST striping so that it is more likely to trigger the code
path that accessed the stale OST list when using O_APPEND layout.

Also clean up code style in the rest of this subtest.

Lustre-change: https://review.whamcloud.com/51602
Lustre-commit: 7bb1685048bf999df03ceadab39faa09b8a5560d

Test-Parameters: trivial testlist=sanity env=ONLY=27M,ONLY_REPEAT=200
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie94e3a32fc48198e4e15f44a55d1f8ccf61c74f5
Reviewed-by: Thomas Bertschinger <bertschinger@lanl.gov>
Reviewed-by: Alex Deiter <alex.deiter@gmail.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52013
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>