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()
+}

idea/tests/org/jetbrains/jet/completion/handlers/CompletionHandlerTest.java

@@ -68,6 +68,10 @@ public class CompletionHandlerTest extends LightCompletionTestCase {
         doTest();
     }
 
+    public void testExtFunction() {
+        doTest();
+    }
+
     public void testHigherOrderFunction() {
         doTest();
     }