fix: add libxtst-dev for SDL3's X11 XTEST extension in editor CI jobs

The previous X11 dev-package fix got CNA's vendored SDL3 past its
"no X11 or Wayland" gate (confirmed on the actual CI run: "-- Found
X11: /usr/include" now appears), but configure still failed one step
later: "Couldn't find dependency package for XTEST." SDL3's X11 backend
also needs the XTest extension's headers (X11/extensions/XTest.h),
provided by libxtst-dev, which the previous package list missed.

Author: robertvokac

.github/workflows/ci.yml

@@ -269,7 +269,7 @@ jobs:
             libgl1-mesa-dev libegl1-mesa-dev libglvnd-dev \
             libvulkan-dev \
             libx11-dev libxext-dev libxrandr-dev libxcursor-dev \
-            libxi-dev libxfixes-dev libxss-dev \
+            libxi-dev libxfixes-dev libxss-dev libxtst-dev \
             libsqlite3-dev libssl-dev libxml2-dev xvfb x11-utils
 
       - name: Cache root CMake FetchContent downloads
@@ -358,7 +358,7 @@ jobs:
             clang cmake ninja-build python3 pkg-config \
             libgl1-mesa-dev libegl1-mesa-dev libglvnd-dev \
             libx11-dev libxext-dev libxrandr-dev libxcursor-dev \
-            libxi-dev libxfixes-dev libxss-dev \
+            libxi-dev libxfixes-dev libxss-dev libxtst-dev \
             libsqlite3-dev libssl-dev libxml2-dev xvfb x11-utils
 
       - name: Cache root CMake FetchContent downloads