tests/extmod/os_urandom.py: Add test for os.urandom.

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

Author: dpgeorge

tests/extmod/os_urandom.py

@@ -0,0 +1,14 @@
+# Test os.urandom().
+
+try:
+    from os import urandom
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+for n in range(-2, 5, 1):
+    try:
+        r = urandom(n)
+        print(n, type(r), len(r))
+    except ValueError:
+        print(n, "ValueError")