Improve unit test coverage for REST API menus endpoints.

[spacedmonkey]

What problem does this address?

Background

The menus endpoints, menu, menu locations and menu items were originally worked on by @spacedmonkey and developed in another github repo. These endpoints, were merged into this plugin in #20292.

Now that the menus endpoints are being used for the menu screen. There have been a number of changes to the REST API endpoint over the last couple of months these changes include.

#31656, #34673, #34888, #34835

Let's audit and improve PHPUnit test coverage for these endpoints.

What is your proposed solution?

[anton-vlasenko]

Let's consider adding these unit tests as well: 5b6c2d2
Discussion can be found here: #34857