Build: use K2 for stdlib tests when enabled

enables testing of #KT-56379

libraries/stdlib/jvm/build.gradle

@@ -201,6 +201,9 @@ compileTestKotlin {
                 "-XXLanguage:+RangeUntilOperator",
                 "-Xallow-kotlin-package", // TODO: maybe rename test packages
         ]
+        if (project.kotlinBuildProperties.useFir) {
+            freeCompilerArgs += "-Xuse-k2"
+        }
         // This is needed for JavaTypeTest; typeOf for non-reified type parameters doesn't work otherwise, for implementation reasons.
         def args = new ArrayList(freeCompilerArgs)
         args.remove("-Xno-optimized-callable-references")