FE: use WITH_STDLIB directive for Enum.entries tests

compiler/testData/diagnostics/tests/enum/entries/genericEntriesPropertyClash.fir.kt

@@ -1,4 +1,5 @@
 // !LANGUAGE: -EnumEntries
+// WITH_STDLIB
 
 package pckg
 
@@ -11,6 +12,6 @@ enum class A {
 val <T> T.entries: Int get() = 0
 
 fun test() {
-    A.entries
+    A.<!OPT_IN_USAGE_ERROR!>entries<!>
     A.Companion.entries
 }