Move callable reference diagnostic tests to non-stdlib

compiler/testData/diagnostics/tests/callableReference/function/extensionOnNullable.kt

@@ -0,0 +1,12 @@
+// !DIAGNOSTICS:-UNUSED_VARIABLE
+
+import kotlin.reflect.*
+
+class A {
+    fun foo() {}
+}
+
+fun A?.foo() {}
+
+val f: KMemberFunction0<A, Unit> = A::foo
+val g: KExtensionFunction0<A, Unit> = A?::foo