Minor, add test on name mangling of private multi-file part members

compiler/testData/codegen/bytecodeText/multifileClasses/privateFunctionName.kt

@@ -0,0 +1,19 @@
+// WITH_RUNTIME
+// FILE: part1.kt
+
+@file:JvmName("Facade")
+@file:JvmMultifileClass
+
+package test
+
+private fun privateFun() {}
+
+private var privateVar = 42
+    get() = field
+    set(value) { field = value }
+
+// We mangle names of private declarations in multi-file parts because in the -Xmultifile-parts-inherit mode, they can clash accidentally.
+// Below, one usage of each declaration is at the declaration site, another in the Metadata.d2 array.
+// 2 privateFun\$Facade__Part1Kt
+// 2 getPrivateVar\$Facade__Part1Kt
+// 2 setPrivateVar\$Facade__Part1Kt