[FIR & IR] Implement JS MPP test infrastructure

^KT-55295 Fixed

compiler/testData/codegen/box/multiplatform/defaultArguments/typeAlias2.kt

@@ -1,7 +1,7 @@
 // !LANGUAGE: +MultiPlatformProjects
 // WITH_STDLIB
 // IGNORE_BACKEND_K1: ANY
-// IGNORE_BACKEND_K2: JS_IR, NATIVE
+// IGNORE_BACKEND_K2: NATIVE
 
 // MODULE: common
 // FILE: common.kt

compiler/testData/codegen/box/multiplatform/multiModule/expectActualSimpleJs.kt

@@ -0,0 +1,28 @@
+// KJS_WITH_FULL_RUNTIME
+// IGNORE_BACKEND_K1: JS, JS_IR, JS_IR_ES6
+
+// TARGET_BACKEND: JS_IR
+// !LANGUAGE: +MultiPlatformProjects
+
+// MODULE: common
+// TARGET_PLATFORM: Common
+// FILE: common.kt
+
+expect fun func(): String
+
+expect var prop: String
+
+fun test(): String {
+    prop = "K"
+    return func() + prop
+}
+
+// MODULE: js()()(common)
+// TARGET_PLATFORM: JS
+// FILE: main.kt
+
+actual fun func(): String = "O"
+
+actual var prop: String = "!"
+
+fun box() = test()