Add test auto-configuration for MockMvcTester

[snicoll]

Spring Framework 6.2 introduces an AssertJ support for MockMvc that also provides a fluent API to compose the request. Given that we're auto-configuring a MockMvc instance in certain circumstances, we should extend that to support the tester as well (which wraps it).

[snicoll]

As part of this change, we'll also migrate our tests to this since we're using AssertJ everywhere.