[K/JS] Add a secret system property to enable the old Kotlin/JS compiler only for tests until it will be removed from the sources

compiler/tests/org/jetbrains/kotlin/cli/LauncherScriptTest.kt

@@ -18,6 +18,7 @@ package org.jetbrains.kotlin.cli
 
 import com.intellij.openapi.util.SystemInfo
 import com.intellij.openapi.util.text.StringUtil
+import org.jetbrains.kotlin.cli.common.CompilerSystemProperties
 import org.jetbrains.kotlin.cli.common.ExitCode
 import org.jetbrains.kotlin.cli.jvm.K2JVMCompiler
 import org.jetbrains.kotlin.test.KotlinTestUtils
@@ -142,7 +143,12 @@ class LauncherScriptTest : TestCaseWithTmpdir() {
         runProcess(
             "kotlinc-js",
             "$testDataDirectory/emptyMain.kt",
-            "-Xlegacy-deprecated-no-warn", "-Xuse-deprecated-legacy-compiler", "-output", File(tmpdir, "out.js").path
+            "-nowarn",
+            "-Xlegacy-deprecated-no-warn",
+            "-Xuse-deprecated-legacy-compiler",
+            "-D${CompilerSystemProperties.KOTLIN_JS_COMPILER_LEGACY_FORCE_ENABLED.property}=true",
+            "-output",
+            File(tmpdir, "out.js").path,
         )
     }