Rename *.jet -> *.kt in codegen tests

compiler/testData/codegen/patternMatching/is.kt

@@ -0,0 +1,11 @@
+fun typeName(a: Any?) : String {
+    return when(a) {
+        is java.util.ArrayList<*> -> "array list"
+        else -> "no idea"
+    }
+}
+
+fun box() : String {
+    if(typeName(java.util.ArrayList<Int> ()) != "array list") return "array list failed"
+    return "OK"
+}