Update `TestRestTemplate` to handle `BasicAuthorizationInterceptor`s
correctly. Prior to this commit the `BasicAuthorizationInterceptor`
was added directly to the `ClientHttpRequestFactory` rather than
to the `RestTemplate`. This meant that it could not easily be removed
when `TestRestTemplate.withBasicAuth` was invoked.

The `TestRestTemplate` now sets the interceptor on `RestTemplate`
directly and relies on the logic in `InterceptingHttpAccessor` to
add it to the `ClientHttpRequestFactory`.

Fixes gh-7812