Add support for using mock clock in cron

[sarsharma]

This PR uses https://github.com/benbjohnson/clock to mock the time. Adding support for using mock clock with cron will be helpful to write test cases and verify cron behaviour.

Similar to this PR https://github.com/robfig/cron/pull/327 but uses a more up to date mock clock

PR also contains fix for flaky unit tests on windows and minor linting fixes