[Wasm] Test infra: support _commonFiles and additional .mjs

Use custom `evalToBoolean` as `eval` and `unsafeCast` are not
 available in Wasm stdlib

js/js.translator/testData/_commonFiles/testUtils.kt

@@ -1,6 +1,9 @@
 package testUtils
 
+@JsName("eval")
+private external fun evalToBoolean(code: String): Boolean
+
 fun isLegacyBackend(): Boolean =
     // Using eval to prevent DCE from thinking that following code depends on Kotlin module.
-    eval("(typeof Kotlin != \"undefined\" && typeof Kotlin.kotlin != \"undefined\")").unsafeCast<Boolean>()
+    evalToBoolean("(typeof Kotlin != \"undefined\" && typeof Kotlin.kotlin != \"undefined\")")