LU-15483 tests: Improve test 398b

LU-15483 tests: Improve test 398b

Test 398b is currently only writing 3 MiB (12 MiB/4) of
data from each thread in the racing set (4 DIO, 4 BIO).
This is such a small amount that it's probably finishing
too quickly to generate the intended overlap & races most
of the time.

It also currently writes only at PAGE_SIZE, which excludes
a lot of possible races.

This is currently a pretty fast test - ~ 2 seconds on my
local machine, similar on Maloo - so let's have it write 4
times more data and use 4 different block sizes, so it will
hit more races.

This is 16x more I/O, but the test still only takes around
16 seconds on my local machine.

This is motivated by difficulties dealing with rare
failures while developing the LU-13799 patch series and
hoping to make them easier to catch.

Test-Parameters: trivial testlist=sanity env=ONLY=398b,ONLY_REPEAT=50
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I1092023a526c085dfacd1bd112c2ebb714da63e5
Reviewed-on: https://review.whamcloud.com/44321
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wangshilong1991@gmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>