docs: Add XTEST library to the Fedora package list

SDL requires that all the X11 libraries be present for configuration to succeed by default, and the package list was missing this one.

docs/README-linux.md

@@ -28,7 +28,7 @@ Fedora 35, all available features enabled:
     sudo dnf install gcc git-core make cmake \
     alsa-lib-devel fribidi-devel pulseaudio-libs-devel pipewire-devel \
     libX11-devel libXext-devel libXrandr-devel libXcursor-devel libXfixes-devel \
-    libXi-devel libXScrnSaver-devel dbus-devel ibus-devel \
+    libXi-devel libXScrnSaver-devel libXtst-devel dbus-devel ibus-devel \
     systemd-devel mesa-libGL-devel libxkbcommon-devel mesa-libGLES-devel \
     mesa-libEGL-devel vulkan-devel wayland-devel wayland-protocols-devel \
     libdrm-devel mesa-libgbm-devel libusb1-devel libdecor-devel \