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

buildSrc/build.gradle.kts

@@ -68,7 +68,7 @@ gradlePlugin {
 }
 
 versionCatalogUpdate {
-    sortByKey.set(false)
-    keep { keepUnusedLibraries.set(true) }
-    catalogFile.set(file("../gradle/libs.versions.toml"))
+    sortByKey = false
+    keep { keepUnusedLibraries = true }
+    catalogFile = file("../gradle/libs.versions.toml")
 }