Stop the filer test helpers from pinning gigabytes of log buffers (#10560)

* log buffer: wake the interval loop on shutdown instead of sleeping through it

loopInterval parked in time.Sleep(flushInterval) and only re-checked
IsStopping when it woke, so a buffer shut down early kept both loop
goroutines - and the PreviousBufferCount+1 slabs of BufferSize they
reach - alive for up to a full interval afterwards. Select on shutdownCh
against a ticker instead, and give the loops a WaitGroup so a test can
observe that they exit.

* test: release the filers the server tests build

Every helper here left its filer's meta log buffer running, so each test
pinned PreviousBufferCount+1 buffers of BufferSize for the rest of the
run: ~3.5GB of live heap across the package, which overruns the address
space on linux/386 and kills the 32-bit job with an out-of-memory throw.

Thread the test through the helpers so the buffer is shut down on
cleanup, and shut the subscribe harness's filer down outright - its
deletion loop keeps the whole filer reachable otherwise. That harness
quiesces its flush path first, since Filer.Shutdown closes the store a
flush still in flight would write through.

weed/server/filer_grpc_server_create_lock_test.go

@@ -18,7 +18,7 @@ import (
 func TestCreateEntryOExclSerialized(t *testing.T) {
 	store := newRenameTestStore()
 	store.findDelay = 5 * time.Millisecond
-	f := newRenameTestFiler(store)
+	f := newRenameTestFiler(t, store)
 	f.DirBucketsPath = "/buckets"
 
 	fs := &FilerServer{