Switch flag definitions to be on pflag instead of flag in `package go/vt/vttablet/tabletconntest`

Part of https://github.com/vitessio/vitess/issues/10697.

Current flags:

```
$ git grep -E "\bflag\.[A-Z]" -- go/vt/vttablet/tabletconntest/*.go
go/vt/vttablet/tabletconntest/tabletconntest.go:                flag.Set("grpc_auth_static_client_creds", clientCreds.Name())
```