Tests: fix stdlib declarations in IR interpreter test data

Fix some unresolved supertypes. This is necessary to be able to enable
IR fake override builder by default (KT-61514), because it traverses all
supertypes and asserts that they're classes, so that it can build fake
overrides for declarations from there. Without this change, for example
`IrFakeOverrideBuilder.buildFakeOverridesForClass` would crash.

compiler/testData/ir/interpreter/helpers/StringNumberConversionsJVM.kt

@@ -0,0 +1,4 @@
+package kotlin.text
+
+@SinceKotlin("1.2")
+public actual inline fun Long.toString(radix: Int): String = TODO()