tests/pybnative: Make while.py test run on boards without pyb.delay.

Signed-off-by: Damien George <damien@micropython.org>

tests/pybnative/while.py

-import pyb
+import time, pyb
 
 
 @micropython.native
@@ -8,7 +8,7 @@ def f(led, n, d):
     while i < n:
         print(i)
         led.toggle()
-        pyb.delay(d)
+        time.sleep_ms(d)
         i += 1
     led.off()