Merge boxWithStdlib testData into box, delete BoxWithStdlib test

compiler/testData/codegen/box/topLevelPrivate/syntheticAccessorInMultiFile.kt

@@ -0,0 +1,15 @@
+// WITH_RUNTIME
+
+@file:kotlin.jvm.JvmMultifileClass
+@file:kotlin.jvm.JvmName("TestKt")
+package test
+
+private val prop = "O"
+
+private fun test() = "K"
+
+fun box(): String {
+    return {
+        prop + test()
+    }()
+}