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/util/log_buffer/log_buffer_test.go

@@ -576,3 +576,24 @@ func TestLoopProcessLogDataWithOffset_DiskReadRetry(t *testing.T) {
 		t.Logf("✓ SUCCESS: Message received after %d disk read attempts", finalDiskReadCount)
 	}
 }
+
+// A buffer shut down long before its flush interval elapses must not keep its
+// loops - and the tens of megabytes of slabs they reach - alive until the next
+// tick would have fired.
+func TestShutdownStopsGoroutinesPromptly(t *testing.T) {
+	lb := NewLogBuffer("shutdown", time.Hour, nil, nil, nil)
+	// let both loops start and park in their waits before shutting down
+	time.Sleep(100 * time.Millisecond)
+	lb.ShutdownLogBuffer()
+
+	done := make(chan struct{})
+	go func() {
+		lb.loopsDone.Wait()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(5 * time.Second):
+		t.Fatal("log buffer goroutines still running 5s after shutdown")
+	}
+}