xds: tests shouldn't rely on the presence of an entry in the authorities field of the bootstrap configuration with an empty key

[easwars]

Currently many tests rely on the presence of a key in the authorities map of the bootstrap config with an empty value. And this has an empty config, resulting in the usage of the top-level xDS server config. Tests should be fixed to not have this assumption.

See:

grpc-go/internal/testutils/xds/bootstrap/bootstrap.go

Line 121 in 5d7bd7a

auths := map[string]authority{"": {}}