Smoke test: truncate the demo server log per run

tests/smoke.sh

@@ -35,6 +35,9 @@ done
 mkdir -p "$RUNS/screenshots"
 LOG="$RUNS/smoke.log"
 : > "$LOG"
+# The player retries the fake demo stream for as long as it is on screen, so
+# without this the request log grows by a few hundred KB per run.
+: > "$RUNS/http.log"
 
 step() { printf '\n== %s\n' "$1" | tee -a "$LOG"; }
 fail() { printf '\nFAIL: %s\n' "$1" | tee -a "$LOG" >&2; exit 1; }