EX-8598 tests: use alternative data source for rewriting

EX-8598 tests: use alternative data source for rewriting

Using the same file as input has disadvantages. It is not
possible to understand that data was not rewritten at all.
Alternative data source should be used.

Let's shift source file data and use it as a source.
To check rewriting result the same operarion is performed
on the destination file copy stored outside the Lustre FS.

Signed-off-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Test-Parameters: trivial testlist=sanity-compr env=ONLY=1004
Change-Id: I6ef400520359bfe9156c3f47e757064863bdf4e0
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53088
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>