tests: use new array init syntax, fix mutability

vlib/crypto/rand/rand.v

@@ -11,7 +11,7 @@ const (
 // NOTE: temp until we have []bytes(buff)
 fn c_array_to_bytes_tmp(len int, buffer voidptr) []byte {
 
-	mut arr := []byte
+	mut arr := []byte{}
 	arr = make(len, 1, 1)
 	arr.data = buffer
 	/*

vlib/crypto/rand/rand_test.v

@@ -31,7 +31,7 @@ fn test_crypto_rand_read() {
 
 fn test_crypto_rand_int_u64() {
 	max := u64(160)
-	mut unique := []int
+	mut unique := []int{}
 	for _ in 0..80 {
 		r := rand.int_u64(max) or {
 			assert false

vlib/crypto/rand/utils.v

@@ -4,10 +4,8 @@
 
 module rand
 
-import(
-	math.bits
-	encoding.binary
-)
+import math.bits
+import encoding.binary
 
 pub fn int_u64(max u64) ?u64 {
 	bitlen := bits.len_64(max)