test-js: Add a mark_as_garbage method to force GC to collect that object

This should fix the flaky tests of test-js.
It also fixes the tests when running with the -g flag since the values
will not be garbage collected too soon.

Userland/Libraries/LibJS/Tests/builtins/FinalizationRegistry/FinalizationRegistry.prototype.cleanupSome.js

@@ -3,11 +3,12 @@ test("length is 0", () => {
 });
 
 function registerInDifferentScope(registry) {
-    registry.register({}, {});
+    const target = {};
+    registry.register(target, {});
+    return target;
 }
 
-// Flaky test, investigate and fix :^)
-test.skip("basic functionality", () => {
+test("basic functionality", () => {
     var registry = new FinalizationRegistry(() => {});
 
     var count = 0;
@@ -19,7 +20,8 @@ test.skip("basic functionality", () => {
 
     expect(count).toBe(0);
 
-    registerInDifferentScope(registry);
+    const target = registerInDifferentScope(registry);
+    markAsGarbage("target");
     gc();
 
     registry.cleanupSome(increment);

Userland/Libraries/LibJS/Tests/builtins/WeakMap/WeakMap.prototype.set.js

@@ -21,10 +21,13 @@ test("invalid values", () => {
 
 test("automatic removal of garbage-collected values", () => {
     const weakMap = new WeakMap();
-    {
-        expect(weakMap.set({ a: 1 }, 1)).toBe(weakMap);
-        expect(getWeakMapSize(weakMap)).toBe(1);
-    }
+    const key = { e: 3 };
+
+    expect(weakMap.set(key, 1)).toBe(weakMap);
+    expect(getWeakMapSize(weakMap)).toBe(1);
+
+    markAsGarbage("key");
     gc();
+
     expect(getWeakMapSize(weakMap)).toBe(0);
 });

Userland/Libraries/LibJS/Tests/builtins/WeakSet/WeakSet.prototype.add.js

@@ -17,10 +17,13 @@ test("invalid values", () => {
 
 test("automatic removal of garbage-collected values", () => {
     const weakSet = new WeakSet();
-    {
-        expect(weakSet.add({ a: 1 })).toBe(weakSet);
-        expect(getWeakSetSize(weakSet)).toBe(1);
-    }
+    const item = { a: 1 };
+
+    expect(weakSet.add(item)).toBe(weakSet);
+    expect(getWeakSetSize(weakSet)).toBe(1);
+
+    markAsGarbage("item");
     gc();
+
     expect(getWeakSetSize(weakSet)).toBe(0);
 });