LU-16348 tests: export TESTLOG_PREFIX and TESTNAME to rpc.sh

LU-16348 tests: export TESTLOG_PREFIX and TESTNAME to rpc.sh

In Lustre test suites, while running do_rpc_nodes, if the
remote function failed and error() was called,
then gather_logs() can not gather logs with a correct
prefix name because TESTLOG_PREFIX and TESTNAME variables
were not exported to rpc.sh.

Test-Parameters: trivial testlist=sanity,conf-sanity
Signed-off-by: Jian Yu <yujian@whamcloud.com>
Change-Id: I2bdbca7f1886f376160a87293ef367f3a4a59f86
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/49260
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>