Fix app config leaking between tests

We need to re-initialise the webauthn_server module with original
app config, since this state is global across all tests. Since the
behaviour of the original fixture wasn't specific to verifying the
origin, I've renamed the fixture as part of making it global.

In order to keep the fixture simple, I've rewritten the test for
the webauthn_server module, so they don't touch the app fixture.

tests/app/main/views/test_webauthn_credentials.py

@@ -2,7 +2,6 @@ import pytest
 from fido2 import cbor
 from flask import url_for
 
-from app import webauthn_server
 from app.models.webauthn_credential import RegistrationError
 
 
@@ -21,18 +20,10 @@ def test_begin_register_returns_encoded_options(
     mocker,
     platform_admin_user,
     platform_admin_client,
+    webauthn_dev_server,
 ):
-    # override base URL so it's consistent on CI and locally
-    mocker.patch.dict(
-        app_.config,
-        values={'ADMIN_BASE_URL': 'http://localhost:6012'}
-    )
-    webauthn_server.init_app(app_)
     mocker.patch('app.user_api_client.get_webauthn_credentials_for_user', return_value=[])
-
-    response = platform_admin_client.get(
-        url_for('main.webauthn_begin_register')
-    )
+    response = platform_admin_client.get(url_for('main.webauthn_begin_register'))
 
     assert response.status_code == 200
 
@@ -50,7 +41,7 @@ def test_begin_register_returns_encoded_options(
 
     relying_party_options = webauthn_options['rp']
     assert relying_party_options['name'] == 'GOV.UK Notify'
-    assert relying_party_options['id'] == 'localhost'
+    assert relying_party_options['id'] == 'webauthn.io'
 
 
 def test_begin_register_includes_existing_credentials(