LibJS: Add test for assignment operators

Libraries/LibJS/Tests/assignment-operators.js

@@ -0,0 +1,41 @@
+load("test-common.js");
+
+try {
+    var x;
+
+    x = 1;
+    assert((x = 2) === 2);
+    assert(x === 2);
+
+    x = 1;
+    assert((x += 2) === 3);
+    assert(x === 3);
+
+    x = 3;
+    assert((x -= 2) === 1);
+    assert(x === 1);
+
+    x = 3;
+    assert((x *= 2) === 6);
+    assert(x === 6);
+
+    x = 6;
+    assert((x /= 2) === 3);
+    assert(x === 3);
+
+    x = 2;
+    assert((x <<= 2) === 8);
+    assert(x === 8);
+
+    x = 8;
+    assert((x >>= 2) === 2);
+    assert(x === 2);
+
+    x = -(2 ** 32 - 10);
+    assert((x >>>= 2) === 2);
+    assert(x === 2);
+
+    console.log("PASS");
+} catch (e) {
+    console.log("FAIL: " + e);
+}