A change [1] in Spring Framework 5.3 means that getUrl() on a
RequestEntity will throw an UnsupportedOperationException if the
entity was created using a template.

This commit updates TestRestTemplate to check for instances of
UriTemplateRequestEntity and to resolve the URI using the
entity's UriTemplateHandler instead of calling getUrl() directly.

Fixes gh-25097

[1] https://github.com/spring-projects/spring-framework/commit/a0f4d81db76cb100b8f0da3a3cc8a945a012a2c0