Added completion handler test with built-in extension function.

idea/testData/completion/handlers/ExtFunction.kt

@@ -0,0 +1,3 @@
+fun main(args : Array<String>) {
+    "".toS<caret>
+}

idea/testData/completion/handlers/ExtFunction.kt.after

@@ -0,0 +1,3 @@
+fun main(args : Array<String>) {
+    "".toString()
+}