Add test toolkit to upgrading guide

Author: willkroboth

docs/en/upgrading-parts/10.1.2-to-11.0.0.md

@@ -22,6 +22,10 @@ commandapi-kotlin-core // [!code ++]
 commandapi-bukkit-kotlin // [!code --]
 commandapi-kotlin-paper // [!code ++]
 commandapi-kotlin-spigot // [!code ++]
+
+commandapi-bukkit-test-toolkit // [!code --]
+commandapi-paper-test-toolkit // [!code --]
+commandapi-spigot-test-toolkit // [!code --]
 ```
 
 :::danger **Developer's Note:**
@@ -36,8 +40,7 @@ There again is no guarantee for any kind of compatibility.
 
 #### Code changes
 
-The `CommandAPIBukkitConfig` class has been converted into an abstract class and is no longer used to construct a config instance. Instead, use the new `CommandAPIPaperConfig` or
-`CommandAPISpigot` classes, depending on the module you use.
+The `CommandAPIBukkitConfig` class has been converted into an abstract class and is no longer used to construct a config instance. Instead, use the new `CommandAPIPaperConfig` or `CommandAPISpigotConfig` classes, depending on the module you use. If you were using `CommandAPIVersionHandler#usePlatformImplementation` from the test toolkit, `MockCommandAPIBukkit` has been changed to `MockCommandAPIPaper` or `MockCommandAPISpigot`. See the new documentation for [that method](/test/load-mock-commandapi#loading-a-custom-command-api-platform-implementation) for details.
 
 Further changes have been made to arguments that work with components. The classes `AdventureChatArgument`, `AdventureChatComponentArgument` and `AdventureChatColorArgument` have been
 removed. Instead, the `ChatArgument`, `ChatComponentArgument` and `ChatColorArgument` have been implemented platform specific and return different types on Paper and Spigot.