Add test for body mixin error

test/bun.js/body-mixin-errors.test.ts

@@ -0,0 +1,17 @@
+import { it, describe, expect } from "bun:test";
+
+describe("body-mixin-errors", () => {
+  it("should fail when bodyUsed", async () => {
+    var res = new Response("a");
+    expect(res.bodyUsed).toBe(false);
+    await res.text();
+    expect(res.bodyUsed).toBe(true);
+
+    try {
+      await res.text();
+      throw new Error("should not get here");
+    } catch (e) {
+      expect(e.message).toBe("Body already used");
+    }
+  });
+});