tests/extmod: Add test for the precision of utime functions.

According to documentation time() has a precision of at least 1 second.
This test runs for 2.5 seconds and calls all utime functions every 100ms.
Then it checks if they returned enough different results.  All functions
with sub-second precision will return ~25 results.  This test passes with
15 results or more.  Functions that do not exist are skipped silently.

tests/extmod/utime_res.py

+# test utime resolutions
+
+try:
+    import utime
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+
+def gmtime_time():
+    return utime.gmtime(utime.time())
+
+
+def localtime_time():
+    return utime.localtime(utime.time())
+
+
+def test():
+    TEST_TIME = 2500
+    EXPECTED_MAP = (
+        # (function name, min. number of results in 2.5 sec)
+        ("time", 3),
+        ("gmtime", 3),
+        ("localtime", 3),
+        ("gmtime_time", 3),
+        ("localtime_time", 3),
+        ("ticks_ms", 15),
+        ("ticks_us", 15),
+        ("ticks_ns", 15),
+        ("ticks_cpu", 15),
+    )
+
+    # call time functions
+    results_map = {}
+    end_time = utime.ticks_ms() + TEST_TIME
+    while utime.ticks_diff(end_time, utime.ticks_ms()) > 0:
+        utime.sleep_ms(100)
+        for func_name, _ in EXPECTED_MAP:
+            try:
+                time_func = getattr(utime, func_name, None) or globals()[func_name]
+                now = time_func()  # may raise AttributeError
+            except (KeyError, AttributeError):
+                continue
+            try:
+                results_map[func_name].add(now)
+            except KeyError:
+                results_map[func_name] = {now}
+
+    # check results
+    for func_name, min_len in EXPECTED_MAP:
+        print("Testing %s" % func_name)
+        results = results_map.get(func_name)
+        if results is None:
+            pass
+        elif func_name == "ticks_cpu" and results == {0}:
+            # ticks_cpu() returns 0 on some ports (e.g. unix)
+            pass
+        elif len(results) < min_len:
+            print(
+                "%s() returns %s result%s in %s ms, expecting >= %s"
+                % (func_name, len(results), "s"[: len(results) != 1], TEST_TIME, min_len)
+            )
+
+
+test()

tests/extmod/utime_res.py.exp

+Testing time
+Testing gmtime
+Testing localtime
+Testing gmtime_time
+Testing localtime_time
+Testing ticks_ms
+Testing ticks_us
+Testing ticks_ns
+Testing ticks_cpu