EX-7601 tests: tests for read-modify-write

EX-7601 tests: tests for read-modify-write

This patch adds tests for the read-modify-write case for
EX-7601.  There's still some additional tests to be added
here, but this is a good start.

Test-Parameters: trivial
Test-Parameters: testlist=sanity-compr env=ONLY="1004 1005",ONLY_REPEAT=10
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I5dd9e566b8274ece99283c8962e0d34225089cc0
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53230
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>