The TestVSphereLogin method still defaults to testing against a real vCenter,
but if the required environment variables are not set, it can test against vcsim.
More tests can be converted to use configFromEnvOrSim(), but can be in follow up PRs.
This refactor is in support of SAML token authentication: #63209
Avoid use of govmomi.Client as it only supports username+password authentication via SessionManager.Login().
Using vim25.Client directly will allow VCP to add other authentication methods,
such as SessionManager.LoginByToken().
