[JS IR BE] Add tests for main function

js/js.translator/testData/box/main/twoMains.kt

@@ -0,0 +1,18 @@
+// EXPECTED_REACHABLE_NODES: 1281
+// CALL_MAIN
+
+var o: String = "fail O"
+var k: String = "K"
+
+fun main() {
+    k = "fail K"
+}
+
+fun main(args: Array<String>) {
+    assertEquals(1, args.size)
+    assertEquals("testArg", args[0])
+
+    o = "O"
+}
+
+fun box() = o + k