LibJS/Tests: Add length test for Temporal.Instant.prototype.round()

Author: https://github.com/linusg
Commit: https://github.com/SerenityOS/serenity/commit/6762378f890
Pull-request: https://github.com/SerenityOS/serenity/pull/9251

Userland/Libraries/LibJS/Tests/builtins/Temporal/Instant/Instant.prototype.round.js

@@ -1,4 +1,8 @@
 describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Instant.prototype.round).toHaveLength(1);
+    });
+
     test("basic functionality", () => {
         const instant = new Temporal.Instant(1111111111111n);
         expect(instant.round({ smallestUnit: "second" }).epochNanoseconds).toBe(1111000000000n);