A few more gametests, update to Gradle 8.12

Okay, listen. I started writing a few more gametests (see #1682), and
then thought I'd do a cheeky Gradle update. However, that broke
vanilla-extract[^1], and also triggered a load of deprecation warnings,
and at that point it was too late to separate the too.

[^1]: 8975ed5a7b

projects/lints/build.gradle.kts

@@ -3,7 +3,7 @@
 // SPDX-License-Identifier: MPL-2.0
 
 plugins {
-    id("cc-tweaked.kotlin-convention")
+    kotlin("jvm")
     id("cc-tweaked.java-convention")
 }