This commit fixes two issues in `TestRestTemplate`:

* it improves the detection of the underlying request factory, using
reflection to look inside the intercepting request factory if
interceptors were configured

* it avoids reusing the same request factory when creating a new
`TestRestTemplate` with `withBasicAuth`. Sharing the same instance would
result in sharing authentication state (HTTP cookies). Since the
original request factory can't be detected consistently, a new one is
selected automatically

See gh-8697