Don't break the "API <= LV" rule in compiler tests

Stability of the language version is used for example to determine
whether the PRE_RELEASE_CLASS diagnostic should be reported. Now that
there are pre-release classes in the standard library (Experimental et
al.), we'd like to avoid this error on their usages in diagnostic tests
which are using the "!API_VERSION: 1.3" directive

compiler/tests-common/tests/org/jetbrains/kotlin/checkers/CompilerTestLanguageVersionSettings.kt

@@ -63,10 +63,12 @@ fun parseLanguageVersionSettings(directiveMap: Map<String, String>): LanguageVer
     val apiVersion = (if (apiVersionString != null) ApiVersion.parse(apiVersionString) else ApiVersion.LATEST_STABLE)
                      ?: error("Unknown API version: $apiVersionString")
 
+    val languageVersion = maxOf(LanguageVersion.LATEST_STABLE, LanguageVersion.fromVersionString(apiVersion.versionString)!!)
+
     val languageFeatures = languageFeaturesString?.let(::collectLanguageFeatureMap).orEmpty()
 
     return CompilerTestLanguageVersionSettings(
-        languageFeatures, apiVersion, LanguageVersion.LATEST_STABLE,
+        languageFeatures, apiVersion, languageVersion,
         mapOf(*listOfNotNull(experimental, useExperimental).toTypedArray())
     )
 }