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

js/js.translator/testData/box/extensionProperty/absExtension.kt

@@ -0,0 +1,11 @@
+package foo
+
+val Double.abs: Double
+    get() = if (this > 0) this else -this
+
+fun box(): String {
+    if (4.0.abs != 4.0) return "fail1";
+    if ((-5.2).abs != 5.2) return "fail2";
+
+    return "OK";
+}