Added BENCH/TEST_PRNG, replacing other ad-hoc sources of randomness

When you add a function to every benchmark suite, you know if should
probably be provided by the benchmark runner itself. That being said,
randomness in tests/benchmarks is a bit tricky because it needs to be
strictly controlled and reproducible.

No global state is used, allowing tests/benches to maintain multiple
randomness stream which can be useful for checking results during a run.

There's an argument for having global prng state in that the prng could
be preserved across power-loss, but I have yet to see a use for this,
and it would add a significant requirement to any future test/bench runner.

runners/test_runner.h

@@ -67,6 +67,12 @@ struct test_suite {
 };
 
 
+// deterministic prng for pseudo-randomness in testes
+uint32_t test_prng(uint32_t *state);
+
+#define TEST_PRNG(state) test_prng(state)
+
+
 // access generated test defines
 intmax_t test_define(size_t define);