consolidate timing code

Refactor stopwatch implementation to do the timing ourselves using our protected performance.now, meaning we can remove the dependency on the unmaintained durations library
Remove the "predictable" implementation of the stopwatch - we used to use it for CCK testing but now don't, so it was only used in one unit test, which here is refactored to tick the clock with sinon in the same way as other tests
Remove beginTiming and endTiming functions and replace with usage of our stopwatch since they were doing the same job

Fixes #2095.

:bank: Refactoring/debt/DX (improvement to code design, tooling, documentation etc. without changing behaviour)

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cucumber / cucumber-js