Tests for slint-lsp

[ogoffart]

There are currently no tests for slint-lsp.

We should have test for auto-completion, or goto definition, and such.

One way to do it would simply to add #[test] in the code with stuff like

#[test]
fn element_name() {
    test_autocompletion(
       "Foo := Ima€ { }"
       &["Image"]
    );
}

(the test_autocompletion funciton removes the € character, but will consider that it is where the cursor is, and will make sure that the autocompletion return the right result)

Another way would be to have a collection of .slint files that contains the cursor position encoded as a comment or so, and have a driver that read these files.

[hunger]

#1570 added the first few tests.