rand.util: add sample_r and sample_nr (#8539)

vlib/rand/util/util_test.v

@@ -0,0 +1,31 @@
+import rand.util
+
+fn test_sample_nr() {
+	lengths := [1, 3, 4, 5, 6, 7]
+	a := ['one', 'two', 'three', 'four', 'five', 'six', 'seven']
+	for length in lengths {
+		b := util.sample_nr(a, length)
+		assert b.len == length
+		for element in b {
+			assert element in a
+			// make sure every element occurs once
+			mut count := 0
+			for e in b {
+				if e == element {
+					count++
+				}
+			}
+			assert count == 1
+		}
+	}
+}
+
+fn test_sample_r() {
+	k := 20
+	a := ['heads', 'tails']
+	b := util.sample_r(a, k)
+	assert b.len == k
+	for element in b {
+		assert element in a
+	}
+}