Tests: minor, improve tests on $EntriesMappings classes

These tests were initially testing that $EntriesMappings classes were
correctly generated, but started to check something else once the
language feature EnumEntries was enabled by default.

compiler/testData/codegen/box/enum/enumEntriesMultimodule.kt

@@ -3,6 +3,7 @@
 // WITH_STDLIB
 
 // MODULE: lib
+// !LANGUAGE: -EnumEntries
 // FILE: MyEnum.kt
 enum class MyEnum {
     Nope, OK

compiler/testData/codegen/box/enum/enumEntriesMultimoduleMultipleMappings.kt

@@ -3,6 +3,7 @@
 // WITH_STDLIB
 
 // MODULE: lib
+// !LANGUAGE: -EnumEntries
 // FILE: MyEnums.kt
 enum class MyEnum {
     N, O

compiler/testData/codegen/box/enum/enumEntriesMultimoduleMultipleMappingsForMultipleClassfiles.kt

@@ -3,6 +3,7 @@
 // WITH_STDLIB
 
 // MODULE: lib
+// !LANGUAGE: -EnumEntries
 // FILE: MyEnums.kt
 enum class MyEnum {
     N, O