LibJS/Tests: Add tests for Date.prototype.toTemporalInstant()

Author: https://github.com/linusg
Commit: 9f2dd89446
Pull-request: https://github.com/SerenityOS/serenity/pull/8580

Userland/Libraries/LibJS/Tests/builtins/Date/Date.prototype.toTemporalInstant.js

@@ -0,0 +1,15 @@
+describe("correct behavior", () => {
+    test("basic functionality", () => {
+        const date = new Date("2021-07-09T01:36:00Z");
+        const instant = date.toTemporalInstant();
+        expect(instant.epochSeconds).toBe(1625794560);
+    });
+});
+
+test("errors", () => {
+    test("this value must be a Date object", () => {
+        expect(() => {
+            Date.prototype.toTemporalInstant.call(123);
+        }).toThrowWithMessage(TypeError, "Not a Date object");
+    });
+});