it: add test for preferred buffer scale

src/it/tests/t0021_preferred_buffer_scale.rs

@@ -0,0 +1,29 @@
+use {
+    crate::it::{test_error::TestResult, testrun::TestRun},
+    std::rc::Rc,
+};
+
+testcase!();
+
+async fn test(run: Rc<TestRun>) -> TestResult {
+    let ds = run.create_default_setup().await?;
+
+    let client = run.create_client().await?;
+
+    let win1 = client.create_window().await?;
+    win1.map2().await?;
+
+    let scale = win1.surface.preferred_buffer_scale.expect()?;
+
+    run.cfg.set_scale(&ds.output, 2.0)?;
+
+    client.sync().await;
+    tassert_eq!(scale.next()?, 2);
+
+    run.cfg.set_scale(&ds.output, 3.0)?;
+
+    client.sync().await;
+    tassert_eq!(scale.next()?, 3);
+
+    Ok(())
+}