EX-7601 tests: add multi-mount compression test

EX-7601 tests: add multi-mount compression test

This adds a multi-mount correctness test for compression.
This races IO from two mountpoints at varying sizes to
stress test compression.

Test-Parameters: trivial
Test-Parameters: testlist=sanity-compr env=ONLY=1006
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: If49cbd6d171068faa802835146f273d835b39bc3
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51842
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>