Simplify GUI tests runner

[GuillaumeGomez]

When running the tests, it now looks like this:

$ cargo test --test gui
   Compiling mdbook v0.5.0-alpha.1 (/home/imperio/rust/mdBook)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.34s
     Running tests/gui/runner.rs (target/debug/deps/gui-9ac52dc99d522383)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.11s
     Running `target/debug/mdbook build /home/imperio/rust/mdBook/test_book`
2025-09-08 17:29:25 [INFO] (mdbook_driver::mdbook): Book building has started
2025-09-08 17:29:25 [INFO] (mdbook_driver::mdbook): Running the html backend
2025-09-08 17:29:26 [INFO] (mdbook_html::html_handlebars::hbs_renderer): HTML book written to `/home/imperio/rust/mdBook/test_book/book`
=> Running 14 doc-ui tests (on 16 threads)...

..............                                     (14/14)


<= doc-ui tests done: 14 succeeded, 0 failed, 0 filtered out

[ehuss]

Thanks!