Consolidate views_ui DisplayTest and DisplayCRUDTest display creation tests

1. The testAddDisplay method in the DisplayTest class tests the creation of a block display, whereas the method in the DisplayCrudTest class tests the page block. It doesn't seem logical to test different displays simultaneously; hence, we could retain only DisplayCrudTest since its class name more aptly suits the test in my opinion.

2. The DisplayCrudTest verifies whether the default block remains unchanged after adding a page block. Is it worthwhile to keep this test?

Regarding the merging with another method: all the methods utilize the straightforward randomView() function, which creates a view with a predefined display. In contrast, we need to add a display using the "Add {display}" button. I haven't found an effective way to merge these functions, so I hope you can.