Stricter check for enum classes in EnumModelProvider #558 (#559)

Co-author: volivan239

Anonymous inner classes declared for enum constants are not enums
themselves, as the necessary condition for being enums is that their
direct supertype is `java.lang.Enum` (see the JavaDoc and comments of
`java.lang.Class#isEnum` method). For these classes, `getEnumConstants`
method returns `null`.

Author: dtim

utbot-fuzzers/src/main/kotlin/org/utbot/fuzzer/providers/EnumModelProvider.kt

@@ -13,7 +13,7 @@ object EnumModelProvider : ModelProvider {
     override fun generate(description: FuzzedMethodDescription): Sequence<FuzzedParameter> = sequence {
         description.parametersMap
             .asSequence()
-            .filter { (classId, _) -> classId.isSubtypeOf(Enum::class.java.id) }
+            .filter { (classId, _) -> classId.jClass.isEnum }
             .forEach { (classId, indices) ->
                 yieldAllValues(indices, classId.jClass.enumConstants.filterIsInstance<Enum<*>>().map {
                     UtEnumConstantModel(classId, it).fuzzed { summary = "%var% = ${it.name}" }