LibJS: Add UTF-16 tests to String.prototype methods that already work

These methods did not require UTF-16 views, so just add test cases to
ensure they remain correct.

This also adds a couple of FIXME comments on tests that will fail even
with UTF-16 String.prototype support (for reasons such as lack of UTF-16
support in RegExp.prototype and Unicode case folding).

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.concat.js

@@ -15,3 +15,10 @@ test("basic functionality", () => {
     expect("".concat(1, {})).toBe("1[object Object]");
     expect("".concat(1, {}, false)).toBe("1[object Object]false");
 });
+
+test("UTF-16", () => {
+    expect("😀".concat()).toBe("😀");
+    expect("😀".concat("a")).toBe("😀a");
+    expect("😀".concat("a", 4)).toBe("😀a4");
+    expect("😀".concat("a", "😀")).toBe("😀a😀");
+});

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.repeat.js

@@ -23,3 +23,9 @@ test("throws correct range errors", () => {
         "foo".repeat(Infinity);
     }).toThrowWithMessage(RangeError, "repeat count must be a finite number");
 });
+
+test("UTF-16", () => {
+    expect("😀".repeat(0)).toBe("");
+    expect("😀".repeat(1)).toBe("😀");
+    expect("😀".repeat(10)).toBe("😀😀😀😀😀😀😀😀😀😀");
+});

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.search.js

@@ -45,3 +45,11 @@ test("override exec with non-function", () => {
     re.exec = 3;
     expect("test".search(re)).toBe(0);
 });
+
+// FIXME: RegExp.prototype [ @@search ] needs to support UTF-16.
+// test("UTF-16", () => {
+//     var s = "😀";
+//     expect(s.search("😀")).toBe(0);
+//     expect(s.search("\ud83d")).toBe(0);
+//     expect(s.search("\ude00")).toBe(1);
+// });

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.toString.js

@@ -4,3 +4,8 @@ test("basic functionality", () => {
     expect("".toString()).toBe("");
     expect("hello friends".toString()).toBe("hello friends");
 });
+
+test("UTF-16", () => {
+    expect("😀".toString()).toBe("😀");
+    expect("😀😀😀".toString()).toBe("😀😀😀");
+});

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.trim.js

@@ -61,4 +61,8 @@ test("multi-byte code point", () => {
     expect("_\u180E".trim()).toBe("_\u180E");
     expect("\u180E".trim()).toBe("\u180E");
     expect("\u180E_".trim()).toBe("\u180E_");
+
+    expect("_😀".trim()).toBe("_😀");
+    expect("😀".trim()).toBe("😀");
+    expect("😀_".trim()).toBe("😀_");
 });