freertos: Add real time stats example

The following commit adds a real time stats example demonstrating how to
obtain task CPU usage statistics over a period of time, as opposed to the current
run time stats in FreeRTOS which measures task CPU usage as a percentage of
total system run time.

examples/system/freertos/real_time_stats/example_test.py

@@ -0,0 +1,31 @@
+from __future__ import print_function
+import os
+import sys
+
+try:
+    import IDF
+except ImportError:
+    # this is a test case write with tiny-test-fw.
+    # to run test cases outside tiny-test-fw,
+    # we need to set environment variable `TEST_FW_PATH`,
+    # then get and insert `TEST_FW_PATH` to sys path before import FW module
+    test_fw_path = os.getenv('TEST_FW_PATH')
+    if test_fw_path and test_fw_path not in sys.path:
+        sys.path.insert(0, test_fw_path)
+    import IDF
+
+STATS_TASK_ITERS = 3
+STATS_TASK_EXPECT = "Real time stats obtained"
+
+
+@IDF.idf_example_test(env_tag='Example_WIFI')
+def test_real_time_stats_example(env, extra_data):
+    dut = env.get_dut('real_time_stats', 'examples/system/freertos/real_time_stats')
+    dut.start_app()
+
+    for iteration in range(0, STATS_TASK_ITERS):
+        dut.expect(STATS_TASK_EXPECT)
+
+
+if __name__ == '__main__':
+    test_real_time_stats_example()