LU-10499 pcc: add test for concurrent read from 2 clients

LU-10499 pcc: add test for concurrent read from 2 clients

This patch add a test case with concurrent read access from 2
clients.
The purpose is to verify that the client will not re-attach file
into PCC backend once attached when the file is read access
concurrently from 2 mount points on a client according to the
PCC attach stats.

EX-bug-id: EX-3730
Test-Parameters: trivial
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ibb038bd3a74f43031b6fab4e65565620c416909e
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54419
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>