crypto.rand: add a test for rand.read/1

vlib/crypto/rand/crypto_rand_read_test.v

@@ -0,0 +1,15 @@
+import crypto.rand
+
+fn test_reading() ? {
+	a := rand.read(32) ?
+	dump(a.hex())
+	assert a.len == 32
+	mut histogram := [256]int{}
+	for b in a {
+		histogram[b]++
+	}
+	dump(histogram)
+	for h in histogram {
+		assert h < 10
+	}
+}