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/bench_runner.c

@@ -529,6 +529,19 @@ void bench_trace(const char *fmt, ...) {
 }
 
 
+// bench prng
+uint32_t bench_prng(uint32_t *state) {
+    // A simple xorshift32 generator, easily reproducible. Keep in mind
+    // determinism is much more important than actual randomness here.
+    uint32_t x = *state;
+    x ^= x << 13;
+    x ^= x >> 17;
+    x ^= x << 5;
+    *state = x;
+    return x;
+}
+
+
 // bench recording state
 static struct lfs_config *bench_cfg = NULL;
 static lfs_emubd_io_t bench_last_readed = 0;