sd/internal/instance: TestCache is flaky

[peterbourgon]

--- FAIL: TestCache (1.00s)
	cache_test.go:68: did not receive expected update
FAIL
FAIL	github.com/go-kit/kit/sd/internal/instance	2.879s

Latest example, though there are many others. @yurishkuro any clue?

[yurishkuro]

looking