build: add simple test_race.sh script

Useful for triggering startup races and can be used with git bisect.

contrib/test_race.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+set -e
+
+if [ "$1" == "--build" ]; then
+    shift
+    echo "building..."
+    zig build -freference-trace --prominent-compile-errors
+fi
+
+for i in {1..60}; do
+    echo "running $i ..."
+    flow --exec quit "$@" || exit 1
+done