Remove the assertion in `filter_rest_url_for_leading_slash()`

[johnbillion]

In ​https://github.com/WordPress/wordpress-develop/pull/1995 I identified some tests which work with faulty assumptions about the data they're testing. While debugging this I noticed that the WP_Test_REST_Controller_Testcase::filter_rest_url_for_leading_slash() method performs an assertion which masks the fact that actual tests are not performing any assertions.

For example the WP_Test_REST_Tags_Controller::test_get_terms_post_args_paging() test is faulty and does not perform any assertions, but this is not reported by PHPUnit because WP_Test_REST_Controller_Testcase::filter_rest_url_for_leading_slash() performs 21 assertions during the test run.

The assertion in WP_Test_REST_Controller_Testcase::filter_rest_url_for_leading_slash() should be converted to a conditional call to fail() so that tests which don't perform any assertions get correctly reported.

[johnbillion]

In 52390:

Build/Test Tools: Remove the assertion in filter_rest_url_for_leading_slash().

This assertion can mask the fact that some other tests don't perform an assertion.

Fixes #54661