Introduce _pytest.timing as a way to control timing during tests

_pytest.timing is an indirection to 'time' functions, which pytest production
code should use instead of 'time' directly.

'mock_timing' is a new fixture which then mocks those functions, allowing us
to write time-reliable tests which run instantly and are not flaky.

This was triggered by recent flaky junitxml tests on Windows related to timing
issues.

src/_pytest/timing.py

@@ -0,0 +1,13 @@
+"""
+Indirection for time functions.
+
+We intentionally grab some "time" functions internally to avoid tests mocking "time" to affect
+pytest runtime information (issue #185).
+
+Fixture "mock_timinig" also interacts with this module for pytest's own tests.
+"""
+from time import perf_counter
+from time import sleep
+from time import time
+
+__all__ = ["perf_counter", "sleep", "time"]