Fix flaky TestDurations test

TestDurations tests the `--durations=N` functionality which reports N
slowest tests, with durations <= 0.005s not shown by default.

The test relies on real time.sleep() (in addition to the code which uses
time.perf_counter()) which makes it flaky and inconsistent between
platforms.

Instead of trying to tweak it more, make it use fake time instead. The
way it is done is a little hacky but seems to work.

src/_pytest/runner.py

@@ -2,8 +2,8 @@
 import bdb
 import os
 import sys
-from time import perf_counter
-from time import time
+from time import perf_counter  # Intentionally not `import time` to avoid being
+from time import time  # affected by tests which monkeypatch `time` (issue #185).
 from typing import Callable
 from typing import Dict
 from typing import List