Use FetchContent to grab latest Catch2 release

The key benefit here is that now we're linking against their CMake
target which makes it easy to change how we depend on Catch2. We
can switch from FetchContent to FindPackage to a git submodule and
never have to change our code because we're depending on Catch2 in
the most flexible way possible.

test/CMakeLists.txt

@@ -1,7 +1,16 @@
+cmake_minimum_required(VERSION 3.14)
+
 set(SRCROOT "${PROJECT_SOURCE_DIR}/test/src")
 
+include(FetchContent)
+FetchContent_Declare(Catch2
+    GIT_REPOSITORY "https://github.com/catchorg/Catch2.git"
+    GIT_TAG v2.13.7)
+FetchContent_MakeAvailable(Catch2)
+
 add_library(sfml-test-main STATIC "${SRCROOT}/CatchMain.cpp")
-target_include_directories(sfml-test-main PUBLIC "${PROJECT_SOURCE_DIR}/extlibs/headers" "${SRCROOT}/TestUtilities")
+target_include_directories(sfml-test-main PUBLIC "${SRCROOT}/TestUtilities")
+target_link_libraries(sfml-test-main PUBLIC Catch2::Catch2)
 target_compile_features(sfml-test-main PRIVATE cxx_std_17)
 
 # System is always built

test/src/CatchMain.cpp

@@ -1,2 +1,2 @@
 #define CATCH_CONFIG_MAIN
-#include <catch.hpp>
+#include <catch2/catch.hpp>

test/src/Network/Packet.cpp

@@ -1,6 +1,6 @@
 #include <SFML/Network.hpp>
 
-#include <catch.hpp>
+#include <catch2/catch.hpp>
 #include <limits>
 
 template <typename IntegerType>

test/src/TestUtilities/SystemUtil.hpp

@@ -6,7 +6,7 @@
 #ifndef SFML_TESTUTILITIES_SYSTEM_HPP
 #define SFML_TESTUTILITIES_SYSTEM_HPP
 
-#include <catch.hpp>
+#include <catch2/catch.hpp>
 
 #include <SFML/System/Vector2.hpp>
 #include <SFML/System/Vector3.hpp>