vfmt fixes

vlib/builtin/string_test.v

@@ -299,3 +299,22 @@ fn test_reverse() {
 	t := ''
 	assert t.reverse() == t
 }
+
+
+struct Foo {
+	bar int 
+} 
+
+fn (f Foo) baz() string {
+	return 'baz' 
+} 
+
+fn test_interpolation() {
+	num := 7 
+	mut s := 'number=$num' 
+	assert s == 'number=7' 
+	foo := Foo{} 
+	s = 'baz=${foo.baz()}' 
+	assert s == 'baz=baz' 
+ 
+}