LU-17009 tests: fix runtests to read file name with backslash

LU-17009 tests: fix runtests to read file name with backslash

If a file in /etc dir has a name with backslash, then runtests
will fail because the read command considers the backslash as
an escape character. This patch fixes the issue by adding "-r"
option to read.

Lustre-change: https://review.whamcloud.com/51847
Lustre-commit: TBD (from 14044bdfe5fa600bd2c2339032ae8bb54e24b5fb)

Test-Parameters: trivial testlist=runtests

Change-Id: Iab912ba9708f5b64e6bb8d8adc266ff23ed32de5
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51869
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sarah Liu <sarah@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>