test/cases: add test case for rand.intn (open-policy-agent#5207)

It looks like we didn't have any before. While it's inherently random, we still
know that for the same inputs, within a single evaluation run, we should only
get one output. So let's assert that.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>
Signed-off-by: Byron Lagrone <byron.lagrone@seqster.com>

test/cases/testdata/rand/test-rand.intn.yaml

@@ -0,0 +1,13 @@
+cases:
+- data:
+  modules:
+  - |
+    package test
+
+    p = count(rands) {
+      rands := { rand.intn("key", 100) | numbers.range(1,100)[_] }
+    }
+  note: rand.intn/consistent values for same arguments
+  query: data.test.p = x
+  want_result:
+  - x: 1