rand: change rand.seed() to receive seed value

vlib/rand/rand_test.v

@@ -0,0 +1,30 @@
+import rand
+
+fn gen_randoms(seed int) []int {
+	mut randoms := [0; 20]
+	rand.seed(seed)
+	for i in 0..20 {
+		randoms[i] = rand.next(100)
+	}
+	return randoms
+}
+
+fn test_rand_reproducibility() {
+	mut randoms1 := gen_randoms(42)
+	mut randoms2 := gen_randoms(42)
+	assert randoms1.len == randoms2.len
+
+	mut len := randoms1.len
+	for i in 0..len {
+		assert randoms1[i] == randoms2[i]
+	}
+
+	randoms1 = gen_randoms(256)
+	randoms2 = gen_randoms(256)
+	assert randoms1.len == randoms2.len
+
+	len = randoms1.len
+	for i in 0..len {
+		assert randoms1[i] == randoms2[i]
+	}
+}