Fix DEPRECATED warning in tests after Mojo v4.50

Output without patch:
t/basic.t ..... Mojo::UserAgent::app_url is DEPRECATED in favor of Mojo::UserAgent::Server::url at t/basic.t line 6
Mojo::UserAgent::app_url is DEPRECATED in favor of Mojo::UserAgent::Server::url at t/basic.t line 7
t/basic.t ..... ok
t/blocking.t .. Mojo::UserAgent::app_url is DEPRECATED in favor of Mojo::UserAgent::Server::url at t/blocking.t line 6
t/blocking.t .. ok
t/live.t ...... ok