tests: boards: intel_adsp/smoke: threshold for CPU timing test

The 4th CPU test tries to gauge tight loop performance. However,
it has been observed occasionally that the calculated would go
over just a bit. The threshold was 3, and the observed ratio
has been bouncing between 2.9 and 3.1. So up the threshold
a little bit to account for that.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

Author: dcpleung

tests/boards/intel_adsp/smoke/src/cpus.c

@@ -135,7 +135,7 @@ static void core_smoke(void *arg)
 	cyc1 = ccount();
 	dt = cyc1 - cyc0;
 	insns = count0 * 2;
-	zassert_true((dt / insns) < 3,
+	zassert_true((dt / insns) < 3.5,
 		     "instruction rate too slow, icache disabled?");
 	printk(" CPI = %d.%2.2d\n", dt / insns, ((1000 * dt) / insns) % 1000);
 }