Minor, move enum diagnostic tests, add another test

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

@@ -0,0 +1,9 @@
+enum class E {
+    E1
+    E2
+}
+
+fun foo() {
+    var <!ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE!>e<!> = E.E1
+    e = <!UNUSED_VALUE!>E.E2<!>
+}