LibJS/Tests: Add tests for Temporal.Instant() constructor

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

@@ -0,0 +1,30 @@
+describe("errors", () => {
+    test("called without new", () => {
+        expect(() => {
+            Temporal.Instant();
+        }).toThrowWithMessage(TypeError, "Temporal.Instant constructor must be called with 'new'");
+    });
+
+    test("argument must be coercible to bigint", () => {
+        expect(() => {
+            new Temporal.Instant(123);
+        }).toThrowWithMessage(TypeError, "Cannot convert number to BigInt");
+        expect(() => {
+            new Temporal.Instant("foo");
+        }).toThrowWithMessage(SyntaxError, "Invalid value for BigInt: foo");
+    });
+});
+
+describe("normal behavior", () => {
+    test("length is 1", () => {
+        expect(Temporal.Instant).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        const instant = new Temporal.Instant(123n);
+        expect(instant.epochNanoseconds).toBe(123n);
+        expect(typeof instant).toBe("object");
+        expect(instant).toBeInstanceOf(Temporal.Instant);
+        expect(Object.getPrototypeOf(instant)).toBe(Temporal.Instant.prototype);
+    });
+});