Minor: add diagnostic test for entries in enum with abstract members

compiler/testData/diagnostics/tests/enum/enumEntryInAbstractEnum.kt

@@ -0,0 +1,13 @@
+enum class EnumClass {
+    E1 {
+        override fun foo() = 1
+        override val bar: String = "a"
+    }
+
+    <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>E2<!> {
+
+    }
+
+    abstract fun foo(): Int
+    abstract val bar: String
+}