LibJS: Reorganize tests into subfolders

Author: https://github.com/mattco98
Commit: 4c48c9d69d
Pull-request: https://github.com/SerenityOS/serenity/pull/2681
Reviewed-by: https://github.com/linusg

Libraries/LibJS/Tests/functions/function-length.js

@@ -0,0 +1,27 @@
+load("test-common.js");
+
+try {
+    function foo() { }
+    assert(foo.length === 0);
+    assert((foo.length = 5) === 5);
+    assert(foo.length === 0);
+
+    function bar(a, b, c) {}
+    assert(bar.length === 3);
+    assert((bar.length = 5) === 5);
+    assert(bar.length === 3);
+
+    function baz(a, b = 1, c) {}
+    assert(baz.length === 1);
+    assert((baz.length = 5) === 5);
+    assert(baz.length === 1);
+
+    function qux(a, b, ...c) {}
+    assert(qux.length === 2);
+    assert((qux.length = 5) === 5);
+    assert(qux.length === 2);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}