bench: add list-empty scenario

bench/scenarios/10-list-empty.sh

@@ -0,0 +1,12 @@
+SCENARIO_DESCRIPTION="distrobox list with no harness containers (user containers may be present)"
+SCENARIO_WARMUP=3
+SCENARIO_RUNS=30
+
+scenario_setup() {
+    local n
+    n=$($EXEC list 2>/dev/null | tail -n +2 | wc -l | tr -d ' ')
+    log_info "  (environment has ~${n} distrobox containers)"
+}
+
+scenario_command() { printf '%s list\n' "$EXEC"; }
+scenario_cleanup() { :; }