arti-bench.sh: set $RUST_LOG if it isn't set.

(An empty $RUST_LOG no output, and confuse the nickm^Wuser.)

tests/chutney/arti-bench.sh

 #!/bin/bash
 set -xe
 
+if [ -z "$RUST_LOG" ]; then
+    echo "Setting RUST_LOG=info for your convenience."
+    export RUST_LOG=info
+fi
+
 target="chutney/networks/basic"
 cd "$(git rev-parse --show-toplevel)"
 [ -d chutney ] || git clone https://gitlab.torproject.org/tpo/core/chutney