flakytest: cache channels closer

[MustafaSaber]

In some tests where we use synctest we create the filter outside becuase we don't care about values in channels and the time. However, in go, chan is attributed to a bubble (synctest) or not in creation time and ctx.Done() channel is only created after the first call to ctx.Done() (lazy loading). So we can have a race where revalidationWorker already started and ran s.ctx.Done() or enqueueRevalidation which run inside the bubble. I the first case the chan will be attributed to no bubble so we can close from outside synctest but in the second case chan will be attributed to the bubble so we can't close from outside.

Using _ := ctx.Done() in main code will force the chan to be always attributed to the routing creating the filter (force loading)

This shows the fix but I think we should't merge because it has no effects outside tests and we can just init all tests inside synctest so it doesn't make sense to change main code to fix the test in this case

The error can be reproduced by:

~ GOMAXPROCS=1 go test ./filters/cache/ -run 'SWR_StaleServedAndRevalidated|AgeHeader_STALE' -count=1000 -race
fatal error: close of synctest channel from outside bubble

goroutine 460 [running]:
context.(*cancelCtx).cancel(0xc00032a7d0, 0x1, {0x105337e98, 0x1053c0900}, {0x0, 0x0})
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/context/context.go:568 +0x134
context.WithCancel.func1()
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/context/context.go:243 +0x54
github.com/zalando/skipper/filters/cache.(*cacheSpec).Close.1()
	/Users/msabertalaat/go/src/skipper/filters/cache/filter.go:147 +0x48
sync.(*Once).doSlow(0xc000339e78, 0xc0003cbc08)
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/sync/once.go:78 +0x98
sync.(*Once).Do(0xc000339e78, 0xc0003cbc08)
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/sync/once.go:69 +0x38
github.com/zalando/skipper/filters/cache.(*cacheSpec).Close(...)
	/Users/msabertalaat/go/src/skipper/filters/cache/filter.go:146
github.com/zalando/skipper/filters/cache.newTestFilterWithCleanup.func2()
	/Users/msabertalaat/go/src/skipper/filters/cache/filter_test.go:47 +0x78
testing.(*common).Cleanup.func1()
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/testing/testing.go:1460 +0x128
testing.(*common).runCleanup(0xc0002fa908, 0x0)
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/testing/testing.go:1820 +0x170
testing.tRunner.func2()
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/testing/testing.go:2187 +0x4c
testing.tRunner(0xc0002fa908, 0x105354a18)
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/testing/testing.go:2199 +0x190
created by testing.(*T).Run in goroutine 1
	/opt/homebrew/Cellar/go/1.27.0/libexec/src/testing/testing.go:2258 +0x7c0
FAIL	github.com/zalando/skipper/filters/cache	2.338s
FAIL

see #4241

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