[#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/flush_test.go

@@ -31,6 +31,7 @@ func TestFlush(t *testing.T) {
 				WithSmallObjectSize(smallSize),
 				WithMetabase(mb),
 				WithBlobstor(bs),
+				WithDisableBackgroundFlush(),
 			}, opts...)...)
 	}