Add ClockMock

[ashleyfrieze]

This is a lot more challenging than it first appears. Part of the challenge is avoiding the user of the library having to do a lot of things or depend on too specific versions of libraries.

• Mockito 3 - has mockStatic but cannot mock System
• Powermockito - depends on Mockito2 so isn't compatible with the mockito3 used natively here
• EasyMock - dead
• AspectJ - perhaps an option though may require some work for users of the library
• JMockit - requires the app to be run with an agent flag
• Instrument - java provides instrumentation options, but none of them fit here

Mocking Instant.now might be possible and Mockito3 could achieve that, though directing most of Instant's calls back to the proper implementation may be a minor challenge.

[ashleyfrieze]

My original idea for this was to provide a ClockStub - see https://github.com/webcompere/system-stubs/commit/2c91fa6156763f74ade7e60f87a2f2f309f5db44 - this would replace time as globally as possible (I'd rather hook into System.currentTimeMillis to get every possible time call). I had the idea of two modes:

• fixed time
• fixed but with an auto increment by a given tick every time the clock is read

While active I'd expect to be able to tell the ClockStub to change the time it returns.