Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/mangling/fun.kt

@@ -0,0 +1,24 @@
+// WITH_RUNTIME
+
+package test
+
+internal fun noMangling() = 1;
+
+class Z {
+    internal fun mangled() = 1;
+}
+
+fun box(): String {
+    val clazz = Z::class.java
+    val declaredMethods = clazz.declaredMethods
+
+    val mangled = declaredMethods.firstOrNull {
+        it.name.startsWith("mangled$")
+    }
+    if (mangled == null) return "Class internal function should exist"
+
+    val topLevel = Class.forName("test.FunKt").getMethod("noMangling")
+    if (topLevel == null) return "Top level internal function should exist"
+
+    return "OK"
+}