Add test for script flag in kotlin.Metadata

It passes at the moment because the test uses old backend, but the
required behavior is not yet supported in JVM IR, and it'll need to be
fixed.

plugins/scripting/scripting-compiler/testData/compiler/metadata_flag.kts

@@ -0,0 +1,3 @@
+class RandomClass
+
+println()

plugins/scripting/scripting-compiler/tests/org/jetbrains/kotlin/scripting/compiler/test/ScriptTest.kt

@@ -13,6 +13,7 @@ import org.jetbrains.kotlin.cli.jvm.compiler.EnvironmentConfigFiles
 import org.jetbrains.kotlin.cli.jvm.compiler.KotlinCoreEnvironment
 import org.jetbrains.kotlin.codegen.CompilationException
 import org.jetbrains.kotlin.config.JVMConfigurationKeys
+import org.jetbrains.kotlin.load.java.JvmAnnotationNames
 import org.jetbrains.kotlin.script.loadScriptingPlugin
 import org.jetbrains.kotlin.scripting.configuration.ScriptingConfigurationKeys
 import org.jetbrains.kotlin.scripting.definitions.KotlinScriptDefinition
@@ -86,6 +87,23 @@ class ScriptTest : TestCase() {
         assertEqualsTrimmed("[(1, a)]", out)
     }
 
+    fun testMetadataFlag() {
+        // Test that we're writing the flag to [Metadata.extraInt] that distinguishes scripts from other classes.
+
+        fun Class<*>.isFlagSet(): Boolean {
+            val metadata = annotations.single { it.annotationClass.java.name == Metadata::class.java.name }
+            val extraInt = metadata.javaClass.methods.single { it.name == JvmAnnotationNames.METADATA_EXTRA_INT_FIELD_NAME }
+            return (extraInt(metadata) as Int) and JvmAnnotationNames.METADATA_SCRIPT_FLAG != 0
+        }
+
+        val scriptClass = compileScript("metadata_flag.kts", StandardScriptDefinition)!!
+        assertTrue("Script class SHOULD have the metadata flag set", scriptClass.isFlagSet())
+        assertFalse(
+            "Non-script class in a script should NOT have the metadata flag set",
+            scriptClass.classLoader.loadClass("Metadata_flag\$RandomClass").isFlagSet()
+        )
+    }
+
     private fun compileScript(
         scriptPath: String,
         scriptDefinition: KotlinScriptDefinition,