test: skip flaky ping test on Windows

Ping test started to fail (in ~70% cases) after Windows CI migration
from Appveyor to GitHub Actions. As a part of this migration, the test
tarantool instance was changed from an instance started on a remote
Linux server to an instance started on the same Windows server under
WSL. Thus, request time has shortened and it supposedly caused the ping
test to fail.

Python documentation [1] declares that

  ...though the time is always returned as a floating point number,
  not all systems provide time with a better precision than 1 second...

Particularly, StackOverflow answer [2] argues that

  For Linux and Mac precision is +- 1 microsecond or 0.001 milliseconds.
  Python on Windows uses +- 16 milliseconds precision due to clock
  implementation problems due to process interrupts.

based on Windows documentation [3].

Assuming that ping requests between the same server services can easily
be under 1 ms, it caused the test to fail.

This patch adds skip for the flaky test, refer to #214 for further
development.

1. https://docs.python.org/3/library/time.html#time.time
2. https://stackoverflow.com/a/1938096/11646599
3. https://docs.microsoft.com/en-us/windows-hardware/drivers/kernel/high-resolution-timers

Follows up #182, part of #214

test/suites/test_dml.py

@@ -146,6 +146,9 @@ def test_05_ping(self):
         # Simple ping test
         # * No exceptions are raised
         # * Ping time > 0
+        if sys.platform.startswith("win"):
+            self.skipTest("Windows clock precision causes test to fail sometimes, see #214")
+
         self.assertTrue(self.con.ping() > 0)
         self.assertEqual(self.con.ping(notime=True), "Success")