What kind of change does this PR introduce?
Adds a test to confirm and conform Refit.HttpClientFactory's AddRefitClient with respect to named HttpClients.

Fixes #1909

What is the current behavior?
AddRefitClient can reuse existing named clients, but it can also reconfigure the named clients' factories.

What is the new behavior?
No new behavior, just a test confirming that existing behavior is to pick up named clients.

What might this PR break?
Nothing

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
Docs could be updated further to make it clear that AddRefitClient will use named clients and that you will reconfigure the named client's factory if you use AddRefitClient's returned IHttpClientBuilder (as described in #1686).