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
db7268b841