cachable: make sure to clear caches between tests #25904

Job	Run time
syntax	53s
formula audit	6m 28s
cask audit	2m 19s
docker	4m 30s
tap syntax	4m 51s
test default formula (Ubuntu 22.04)	4m 21s
tests (online)	8m 13s
test default formula (Ubuntu 20.04)	5m 7s
update-test (Ubuntu 22.04)	1m 2s
tests (generic OS)	5m 53s
vendored gems	45s
test default formula (macOS 13 x86_64)	2m 50s
update-test (macOS 13)	49s
tests (Ubuntu 22.04)	5m 57s
test default formula (macOS 14 arm64)	1m 9s
tests (Ubuntu 20.04)	6m 19s
tests (macOS 13 x86_64)	7m 3s
tests (macOS 14 arm64)	8m 30s
	1h 16m 59s

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