string: fix bug and add tests for string.count

vlib/builtin/string.v

@@ -462,7 +462,7 @@ pub fn (s string) count(substr string) int {
 	mut n := 0
 	mut i := 0
 	for {
-		i := s.index_after(substr, i)
+		i = s.index_after(substr, i)
 		if i == -1 {
 			return n
 		}

vlib/builtin/string_test.v

@@ -338,3 +338,13 @@ fn test_bytes_to_string() {
 	bytes := [`h`, `e`, `l`, `l`, `o`] 
 	assert string(bytes, 5) == 'hello' 
 } 
+
+fn test_count() {
+	assert ''.count('') == 0
+	assert ''.count('a') == 0
+	assert 'a'.count('') == 0
+	assert 'aa'.count('a') == 2
+	assert 'aa'.count('aa') == 1
+	assert 'aabbaa'.count('aa') == 2
+	assert 'bbaabb'.count('aa') == 1
+}