JS: create new common directory for all generated tests, migrate several tests there

js/js.translator/testData/box/inheritance/valOverride.kt

@@ -0,0 +1,15 @@
+package foo
+
+open class C() {
+    open val a = 1
+}
+
+class D() : C() {
+    override val a = 2
+}
+
+fun box(): String {
+    val d: C = D()
+    if (d.a != 2) return "fail: ${d.a}"
+    return "OK"
+}