Previously `TestRestTemplate` would override the configured
`ClientHttpRequestFactory` if the Apache HTTP client library was on
classpath.

This commit fixes two issues:

1. The existing `ClientHttpRequestFactory` is overridden *only* if it is
using the Apache HTTP client variant, in order to wrap it with the
`TestRestTemplate` custom support

2. Calling `withBasicAuth` will no longer directly use the request
factory returned by the internal `RestTemplate`; if client interceptors
are configured, the request factory is wrapped with an
`InterceptingClientHttpRequestFactory`. If we don't unwrap it,
interceptors are copied/applied twice in the newly created
`TestRestTemplate` instance. For that, we need to use reflection as the
underlying request factory is not accessible directly.

Closes gh-8697