Allow MockMvcTester to prepare the query without the need of static imports

[snicoll]

MockMvcTester has been introduced in #21178 and we focused on making sure that the AssertJ support offers a declarative API when asserting on the results.

We are still relying on static imports for building the query itself. While we want to keep a way for users to do that if they so desire, we want also to offer a more declarative way of building the query.