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.lock

@@ -580,6 +580,18 @@ dependencies = [
  "wayland-client",
 ]
 
+[[package]]
+name = "calloop-wayland-source"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "876a7a1dbbe026a55ef47a500b123af5a9a0914520f061d467914cf21be95daf"
+dependencies = [
+ "calloop 0.14.1",
+ "rustix 0.38.38",
+ "wayland-backend",
+ "wayland-client",
+]
+
 [[package]]
 name = "cc"
 version = "1.1.31"
@@ -821,6 +833,25 @@ dependencies = [
  "cfg-if",
 ]
 
+[[package]]
+name = "crossbeam-deque"
+version = "0.8.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9dd111b7b7f7d55b72c0a6ae361660ee5853c9af73f70c3c2ef6858b950e2e51"
+dependencies = [
+ "crossbeam-epoch",
+ "crossbeam-utils",
+]
+
+[[package]]
+name = "crossbeam-epoch"
+version = "0.9.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5b82ac4a3c2ca9c3460964f020e1402edd5753411d7737aa39c3714ad1b5420e"
+dependencies = [
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "crossbeam-utils"
 version = "0.8.20"
@@ -2410,6 +2441,7 @@ dependencies = [
  "bitflags 2.6.0",
  "bytemuck",
  "calloop 0.14.1",
+ "calloop-wayland-source 0.4.0",
  "clap",
  "directories",
  "drm-ffi",
@@ -2435,6 +2467,7 @@ dependencies = [
  "profiling",
  "proptest",
  "proptest-derive",
+ "rayon",
  "sd-notify",
  "serde",
  "serde_json",
@@ -2445,6 +2478,7 @@ dependencies = [
  "tracy-client",
  "url",
  "wayland-backend",
+ "wayland-client",
  "wayland-scanner",
  "xcursor",
  "xshell",
@@ -3337,6 +3371,26 @@ version = "0.6.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "20675572f6f24e9e76ef639bc5552774ed45f1c30e2951e1e99c59888861c539"
 
+[[package]]
+name = "rayon"
+version = "1.10.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b418a60154510ca1a002a752ca9714984e21e4241e804d32555251faf8b78ffa"
+dependencies = [
+ "either",
+ "rayon-core",
+]
+
+[[package]]
+name = "rayon-core"
+version = "1.12.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1465873a3dfdaa8ae7cb14b4383657caab0b3e8a0aa9ae8e04b044854c8dfce2"
+dependencies = [
+ "crossbeam-deque",
+ "crossbeam-utils",
+]
+
 [[package]]
 name = "redox_syscall"
 version = "0.4.1"
@@ -3725,7 +3779,7 @@ checksum = "3457dea1f0eb631b4034d61d4d8c32074caa6cd1ab2d59f2327bd8461e2c0016"
 dependencies = [
  "bitflags 2.6.0",
  "calloop 0.13.0",
- "calloop-wayland-source",
+ "calloop-wayland-source 0.3.0",
  "cursor-icon",
  "libc",
  "log",