[#667] writecache: Fix flush test

Allow to disable background flusher for testing purposes.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

pkg/local_object_storage/writecache/writecachebbolt/options.go

@@ -44,6 +44,8 @@ type options struct {
 	openFile func(string, int, fs.FileMode) (*os.File, error)
 	// metrics is metrics implementation
 	metrics writecache.Metrics
+	// disableBackgroundFlush is for testing purposes only.
+	disableBackgroundFlush bool
 }
 
 // WithLogger sets logger.
@@ -155,3 +157,10 @@ func WithMetrics(metrics writecache.Metrics) Option {
 		o.metrics = metrics
 	}
 }
+
+// WithDisableBackgroundFlush disables background flush, for testing purposes only.
+func WithDisableBackgroundFlush() Option {
+	return func(o *options) {
+		o.disableBackgroundFlush = true
+	}
+}