Add a system for client-side tests (#1219)

- Add a new ClientJavaExec Gradle task, which is used for client-side
   tests. This:

   - Copies the exec spec from another JavaExec task.
   - Sets some additional system properties to configure on gametest framework.
   - Runs Java inside an X framebuffer (when available), meaning we
     don't need to spin up a new window.

   We also configure this task so that only one instance can run at
   once, meaning we don't spawn multiple MC windows at once!

 - Port our 1.16 client test framework to 1.19. This is mostly the same
   as before, but screenshots no longer do a golden test: they /just/
   write to a folder. Screenshots are compared manually afterwards.

   This is still pretty brittle, and there's a lot of sleeps scattered
   around in the code. It's not clear how well this will play on CI.

 - Roll our own game test loader, rather than relying on the mod loader
   to do it for us. This ensures that loading is consistent between
   platforms (we already had to do some hacks for Forge) and makes it
   easier to provide custom logic for loading client-only tests.

 - Run several client tests (namely those involving monitor rendering)
   against Sodium and Iris too. There's some nastiness here to set up
   new Loom run configurations and automatically configure Iris to use
   Complementary Shaders, but it's not too bad. These tests /don't/ run
   on CI, so it doesn't need to be as reliable.

projects/forge/build.gradle.kts

@@ -101,6 +101,8 @@ minecraft {
             workingDirectory(file("run/testClient"))
             parent(client.get())
             configureForGameTest()
+
+            property("cctest.tags", "client,common")
         }
 
         val gameTestServer by registering {
@@ -254,18 +256,32 @@ val runGametest by tasks.registering(JavaExec::class) {
     group = LifecycleBasePlugin.VERIFICATION_GROUP
     description = "Runs tests on a temporary Minecraft instance."
     dependsOn("cleanRunGametest")
+    usesService(MinecraftRunnerService.get(gradle))
 
     // Copy from runGameTestServer. We do it in this slightly odd way as runGameTestServer
     // isn't created until the task is configured (which is no good for us).
     val exec = tasks.getByName<JavaExec>("runGameTestServer")
     dependsOn(exec.dependsOn)
     exec.copyToFull(this)
+
+    systemProperty("cctest.gametest-report", project.buildDir.resolve("test-results/$name.xml").absolutePath)
 }
-
 cct.jacoco(runGametest)
-
 tasks.check { dependsOn(runGametest) }
 
+val runGametestClient by tasks.registering(ClientJavaExec::class) {
+    description = "Runs client-side gametests with no mods"
+    copyFrom("runTestClient")
+    tags("client")
+}
+cct.jacoco(runGametestClient)
+
+tasks.register("checkClient") {
+    group = LifecycleBasePlugin.VERIFICATION_GROUP
+    description = "Runs all client-only checks."
+    dependsOn(runGametestClient)
+}
+
 // Upload tasks
 
 val publishCurseForge by tasks.registering(TaskPublishCurseForge::class) {