Use the non-deprecated SimpleIntervalClock fake kubeclock

[misberner]

The code as well as the test only require a PassiveClock, i.e., a clock that only supports querying the current time, without the ability to schedule future tasks in the style of time.After. That allows the use of a SimpleIntervalClock, which is not deprecated.

Note: IntervalClock doesn't do anything more than SimpleIntervalClock, as all the methods not implemented by SimpleIntervalClock would just panic anyway.

[coveralls]

Pull Request Test Coverage Report for Build 3173107206

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.223%

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Totals
Change from base Build 3067817681:	0.0%
Covered Lines:	1631
Relevant Lines:	1828

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💛 - Coveralls

[everettraven]

Thanks for the contribution @misberner ! The change looks good to me.

/lgtm