Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/jvmOverloads/extensionMethod.kt

@@ -0,0 +1,13 @@
+// WITH_RUNTIME
+
+class C {
+}
+
+@kotlin.jvm.JvmOverloads fun C.foo(o: String, k: String = "K"): String {
+    return o + k
+}
+
+fun box(): String {
+    val m = C::class.java.getClassLoader().loadClass("ExtensionMethodKt").getMethod("foo", C::class.java, String::class.java)
+    return m.invoke(null, C(), "O") as String
+}