string_test: fix randomly failed test

* memory should be initialized with zeros before creating string
* there is no string([]array) constructor, string(byteptr) does not count length correctly, using string(byteptr, len) to fix

vlib/builtin/string_test.v

@@ -327,7 +327,7 @@ fn test_interpolation() {
 } 
 
 fn test_bytes_to_string() {
-	mut buf := malloc(10) 
+	mut buf := calloc(10)
 	buf[0] = `h` 
 	buf[1] = `e` 
 	buf[2] = `l` 
@@ -336,5 +336,5 @@ fn test_bytes_to_string() {
 	assert string(buf) == 'hello' 
 	assert string(buf, 2) == 'he' 
 	bytes := [`h`, `e`, `l`, `l`, `o`] 
-	assert string(bytes) == 'hello' 
+	assert string(bytes, 5) == 'hello' 
 }