JS: tests for KT-7357: extension functions from external KJS library couldn't be called in some cases

libraries/examples/kotlin-js-library-example/src/main/kotlin/sample/Main.kt

@@ -1,8 +1,11 @@
 package library.sample
 
+import kotlin.js.Date
+
 public fun pairAdd(p: Pair<Int, Int>): Int = p.first + p.second
 
 public fun pairMul(p: Pair<Int, Int>): Int = p.first * p.second
 
 public data class IntHolder(val value: Int)
 
+public fun Date.extFun(): Int = 1000