test/py: add a test for the sleep command

Execute "sleep", and validate that it sleeps for approximately the correct
amount of time.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>

test/py/tests/test_sleep.py

+# Copyright (c) 2016, NVIDIA CORPORATION. All rights reserved.
+#
+# SPDX-License-Identifier: GPL-2.0
+
+import pytest
+import time
+
+def test_sleep(u_boot_console):
+    '''Test the sleep command, and validate that it sleeps for approximately
+    the correct amount of time.'''
+
+    # Do this before we time anything, to make sure U-Boot is already running.
+    # Otherwise, the system boot time is included in the time measurement.
+    u_boot_console.ensure_spawned()
+
+    # 3s isn't too long, but is enough to cross a few second boundaries.
+    sleep_time = 3
+    tstart = time.time()
+    u_boot_console.run_command('sleep %d' % sleep_time)
+    tend = time.time()
+    elapsed = tend - tstart
+    delta_to_expected = abs(elapsed - sleep_time)
+    # 0.25s margin is hopefully enough to account for any system overhead.
+    assert delta_to_expected < 0.25