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.

Change-Id: Iab912ba9708f5b64e6bb8d8adc266ff23ed32de5
Test-Parameters: trivial testlist=runtests
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51847
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sarah Liu <sarah@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>