Implement client-server test infra and window opening tests

These tests make a real Niri instance and real Wayland clients (via manual
wayland-rs implementation), both on the same event loop local to the test. This
allows testing the full Wayland interaction, including arbitrary event ordering
and delays.

To start off, add a massive powerset test for the settings that influence where
a window may open.

Cargo.toml

@@ -107,9 +107,12 @@ features = [
 
 [dev-dependencies]
 approx = "0.5.1"
+calloop-wayland-source = "0.4.0"
 insta.workspace = true
 proptest = "1.5.0"
 proptest-derive = { version = "0.5.0", features = ["boxed_union"] }
+rayon = "1.10.0"
+wayland-client = "0.31.7"
 xshell = "0.2.6"
 
 [features]