Minor, add test for obsolete issue

#KT-12843 Obsolete

compiler/testData/diagnostics/tests/callableReference/bound/kt12843.kt

@@ -0,0 +1,6 @@
+class Foo {
+    fun bar() {}
+    fun f() = <!UNRESOLVED_REFERENCE!>Unresolved<!>()::bar
+}
+
+val f: () -> Unit = <!UNRESOLVED_REFERENCE!>Unresolved<!>()::<!UNRESOLVED_REFERENCE!>foo<!>

compiler/testData/diagnostics/tests/callableReference/bound/kt12843.txt

@@ -0,0 +1,12 @@
+package
+
+public val f: () -> kotlin.Unit
+
+public final class Foo {
+    public constructor Foo()
+    public final fun bar(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    public final fun f(): kotlin.reflect.KFunction0<kotlin.Unit>
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}