Fix CSharp269Tests.TestUploadAndDownload GridFs test on sharded cluster

Lately, we started to see this flighty failing https://evergreen.mongodb.com/task_log_raw/dot_net_driver_unsecure_tests_posix__version~4.4_os~ubuntu_1804_topology~sharded_cluster_auth~noauth_ssl~nossl_test_netstandard21_patch_85a95c33aace3d68ac29c960a7dc27de4940afad_60257e130305b9597258fe4f_21_02_11_18_57_23/0?type=T#L1533 quite often on EG including old ones. We need to investigate the reasons for this