tests/kernel: Cast time difference to int32_t before abs call

Clang complains when an unsigned value is passed to abs, even though there
is an implicit cast to a signed type. Insert an explicit cast to make clang
happy.

Signed-off-by: Keith Packard <keithp@keithp.com>

tests/kernel/context/src/main.c

@@ -239,7 +239,7 @@ static void _test_kernel_cpu_idle(int atomic)
 			k_cpu_idle();
 		}
 		dt = k_uptime_ticks() - t0;
-		zassert_true(abs(dt - dur) <= slop,
+		zassert_true(abs((int32_t) (dt - dur)) <= slop,
 			     "Inaccurate wakeup, idled for %d ticks, expected %d",
 			     dt, dur);
 	}