fix tests

vlib/builtin/array_test.v

@@ -16,7 +16,7 @@ fn test_ints() {
 	assert a[4] == 4
 	assert a.last() == 4
 
-	mut s := a.str()
+	s := a.str()
 	assert s == '[1, 5, 2, 3, 4]'
 	assert a[1] == 5
 	assert a.last() == 4

vlib/builtin/string_test.v

@@ -140,7 +140,7 @@ fn test_clone() {
 	mut a := 'a'
 	a += 'a'
 	a += 'a'
-	mut b := a
+	b := a
 	mut c := a.clone()
 	assert c == a
 	assert c == 'aaa'