rand: move the shuffle function from arrays to rand.util (#8587)

vlib/rand/util/util_test.v

@@ -1,3 +1,4 @@
+import rand
 import rand.util
 
 fn test_sample_nr() {
@@ -29,3 +30,28 @@ fn test_sample_r() {
 		assert element in a
 	}
 }
+
+fn test_shuffle() {
+	rand.seed([u32(1), 2]) // set seed to produce same results in order
+	a := [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+	mut b := a.clone()
+	mut c := a.clone()
+	util.shuffle(mut b, 0)
+	util.shuffle(mut c, 0)
+	assert b == [6, 4, 5, 1, 9, 2, 10, 3, 8, 7]
+	assert c == [1, 6, 5, 8, 7, 2, 10, 9, 3, 4]
+	// test shuffling a slice
+	mut d := a.clone()
+	util.shuffle(mut d[..5], 0)
+	assert d == [5, 2, 1, 3, 4, 6, 7, 8, 9, 10]
+	assert d[5..] == a[5..]
+	// test shuffling n items
+	mut e := a.clone()
+	util.shuffle(mut e, 5)
+	assert e[..5] == [10, 3, 1, 8, 4]
+	assert e[5..] == [6, 7, 5, 9, 2]
+	// test shuffling empty array
+	mut f := a[..0]
+	util.shuffle(mut f, 0)
+	assert f == []int{}
+}