Add function to moodlelib.php to get the current time to allow it to be changed for unit tests.

Add current_time() to moodlelib - which either just returns time() or if a global is set returns the time relative to a manually set time.

Add a function to the data generator to allow the current time to be set.

Replace all calls to time() with current_time().

Concerns

• current_time() is not very descriptive
• changing all calls to time() will create a large patch (1232 instances outside of tests)