tests: test string literal indexing and mutable array filter

vlib/builtin/array_test.v

@@ -497,6 +497,10 @@ fn test_filter() {
 	println(arr.filter(it % 2 == 0 || it % 3 == 0))
 	assert true
 	assert [1,2,3].len == 3
+	mut mut_arr := [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+	mut_arr = mut_arr.filter(it < 4)
+	assert mut_arr.len == 3
+
 	// TODO
 	//assert arr.filter(arr % 2).len == 5
 }

vlib/builtin/string_test.v

@@ -344,6 +344,8 @@ fn test_lower() {
 	assert s.to_lower() == 'have a nice day!'
 	s = 'hi'
 	assert s.to_lower() == 'hi'
+	assert 'aloha!'[0] == `a`
+	assert 'aloha!'[5] == `!`
 }
 
 fn test_upper() {