test: clear tap instance caches between test runs again

[apainintheneck]

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#16746 (comment)

The core taps exist outside of the normal cache busting cycle so they need to clear explicitly at the instance level. In theory all the other tap instances should get cleared by the class level purge of the Tap#fetch cache. Just to be sure we should clear all of them each time.

This essentially reverts a small regression in this PR.

• cachable: make sure to clear caches between tests #16746

CC: @reitermarkus

[MikeMcQuaid]

Thanks again @apainintheneck!