Enable //gfe/gfe2/quic:quic_backend_connection_test in gfe_cb_feature_flags.
Enable //gfe/gfe2/quic:quic_backend_connection_test in gfe_cb_feature_flags.

gfe-relnote: (n/a) Test only change.
PiperOrigin-RevId: 302111179
Change-Id: Idf2e87029660e3f84e5e722cda42f4fdacd24071

google/quiche,google/quiche,google/quiche,google/quiche