If `FIREBASE_AUTH_EMULATOR_HOST` is set, `VerifyIDToken` calls the Firebase Emulator, but shouldn't

### [REQUIRED] Step 2: Describe your environment

* Operating System version: no matter
* Firebase SDK version: firebase.google.com/go/v4 v4.10.0
* Library version: v4.10.0
* Firebase Product: auth

### [REQUIRED] Step 3: Describe the problem

I am working on functionality using AdminSDK to verify ID tokens by the `VerifyIDToken` function. I would like to write tests including calls of the method, but I found that the SDK tries to call a Firebase Emulator, which host is defined by `FIREBASE_AUTH_EMULATOR_HOST` variable.

I was surprised because I was sure `VerifyIDToken` shouldn't call the emulator to check user revocation. I found [the place](https://github.com/firebase/firebase-admin-go/blob/master/auth/auth.go#L338) where the problem happens. Could you please describe the reason for this behavior or help me to avoid calls to the emulator?

#### Relevant Code:

```go

// IDToken returns a valid ID token
func IDToken(uid string) (string, error) {
const projectID = "project-id"

var headers = map[string]interface{}{
"alg": "none",
"typ": "JWT",
}

jsonHeaders, err := json.Marshal(headers)
if err != nil {
return "", err
}
encodedHeaders := jwt.EncodeSegment(jsonHeaders)

var claims = map[string]interface{}{
"aud": projectID,
"iss": "https://securetoken.google.com/" + projectID,
"iat": time.Now().Unix() - 100,
"exp": time.Now().Unix() + 3600,
"auth_time": time.Now().Unix() - 100,
"sub": "1234567890",
"uid": uid,
}

jsonClaims, err := json.Marshal(claims)
if err != nil {
return "", err
}
encodedClaims := jwt.EncodeSegment(jsonClaims)

return encodedHeaders + "." + encodedClaims + ".", nil
}

func check() {
ctx := context.Background()
token, _ := IDToken("uid")
os.Setenv("FIREBASE_AUTH_EMULATOR_HOST", "localhost:9099")

app, _ := firebase.NewApp(ctx, nil)
auth, _ := app.Auth(ctx)
t, err := auth.VerifyIDToken(ctx, token)
if err != nil {
fmt.Println(err)
}
}
```

I get this error
```
failed to establish a connection: Post \"http://localhost:9099/identitytoolkit.googleapis.com/v1/projects/project-id/accounts:lookup\": dial tcp [::1]:9099: connect: connection refused"
```

Thank you!