fix: run cargo test under xvfb-run; clean up unused imports and mut warning

ahfail_tests.rs calls on_activation which initialises GTK — needs a virtual
display in CI just like the Meson test step.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/ahfail-gtklock/tests/ahfail_tests.rs

@@ -241,7 +241,7 @@ fn run_test_09_idle_hide_cleanup() {
         flush_events();
         
         // Mock GtkLock struct
-        let mut windows_array = glib::ffi::g_array_new(0, 0, std::mem::size_of::<*mut Window>() as u32);
+        let windows_array = glib::ffi::g_array_new(0, 0, std::mem::size_of::<*mut Window>() as u32);
         glib::ffi::g_array_append_vals(windows_array, &ctx_ptr as *const _ as *const c_void, 1);
         
         let mut lock = GtkLock {

crates/ahfail-gtklock/tests/benchmarks.rs

@@ -1,5 +1,4 @@
-use gtk::prelude::*;
-use gtk::{gdk, gdk_pixbuf};
+use gtk::gdk_pixbuf;
 use gstreamer as gst;
 use gstreamer_player as gst_player;
 use std::time::Instant;