testserver: make sure we don't overwrite the NAME variable set

This fixes some oddities stopping and starting servers

fstest/testserver/init.d/run.bash

@@ -2,7 +2,7 @@
 set -euo pipefail
 
 BASE="${STATE_DIR:-${XDG_RUNTIME_DIR:-/tmp}/rclone-test-server}"
-NAME="$(basename "$0")"
+: "${NAME:=$(basename "$0")}"
 ROOT="${BASE}/${NAME}"
 STATE="${ROOT}/state"
 LOCKF="${ROOT}/lock"