[CLI] Don't suggest enabling test-only features

This commit changes the error message from:
    main.kt:2:11: error: the feature "references to synthetic java properties" is experimental and should be enabled explicitly. This can be done by supplying the compiler argument '-XXLanguage:+ReferencesToSyntheticJavaProperties', but note that no stability guarantees are provided.

to
    main.kt:2:11: error: the feature "references to synthetic java properties" is unsupported.

compiler/frontend.common-psi/src/org/jetbrains/kotlin/diagnostics/rendering/LanguageFeatureMessageRenderer.kt

@@ -38,6 +38,8 @@ class LanguageFeatureMessageRenderer @JvmOverloads constructor(
                 when {
                     settings.supportsFeature(feature) && settings.languageVersion < LanguageVersion.KOTLIN_2_0 ->
                         sb.append("not supported in language versions 1.*, please use version 2.0 or later")
+                    feature.kind.testOnly ->
+                        sb.append("unsupported.")
                     since == null ->
                         sb.append("experimental and should be enabled explicitly. This can be done by supplying the compiler argument '$featureFlag', but note that no stability guarantees are provided.")
                     since > settings.languageVersion ->