LU-16541 tests: Improve test 64f

LU-16541 tests: Improve test 64f

The buffered IO part of test 64f has several timing related
holes and other oddities.  The use of multiop in the
background does not guarantee the RPC will not be sent, AND
the test doesn't kill it correctly.

Clean this up and make a more reliable version of the test.
Hopefully this will resolve the failure issues, if not, a
better version of the test will allow debugging.

Lustre-commit: 33e4d86a480b860e0a3b4b51c7c6da6ec0159e51
Lustre-change: https://review.whamcloud.com/52040

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I25b825e1d9d516635ef8cbd26dd12809625c34df
Reviewed-by: xinliang <xinliang.liu@linaro.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52096
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>